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I know that I can have virtualization software (Virtual Box, Virtual PC) running on Ubuntu or Windows and run guest operating systems in it. Is it possible to run virtual machines without a host OS?

Thanks in advance. I am not very familiar with virtualization. Sorry if it is a basic question. I couldn't find an answer on Google. If you can point me to something free that would be great!

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look ate the server core in windows server 2008 –  Saif Khan Jun 10 '09 at 22:57
    
It's a great idea, or another solution, for example, we make a minimal windows xp host as drivers' container and run any OS on it. –  diyism Sep 26 '12 at 7:40
    
All answers are wrong because per definition a hypervisor IS a host OS. So, unless you magically start a VM without anything before (which asks: what loads the VM) you always run a OS. –  TomTom Jun 16 at 10:15
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10 Answers 10

A couple of free, bare-metal virtualization software options:

  • VMWare ESXi
  • Microsoft Hyper-V Server 2008
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Hyper-V is not free. –  Josh Brower Jun 11 '09 at 2:03
    
@Anaplogetos, The link above says "free download." My understanding is that Microsoft used to charge $28 per server for Hyper-V Server 2008, but made it free about a month after VMWare made ESXi free. computerworld.com/action/… Am I missing something? –  Pete TerMaat Jun 11 '09 at 5:10
    
Pete's right... it's free. The link didn't come through on his post for some reason, but it can be downloaded here: microsoft.com/hyper-v-server/en/us/default.aspx –  Sean Earp Jun 11 '09 at 5:26
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http://www.xen.org/ for linux?

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ServerFault is really oriented VMWare/Hyper-V ! –  Antoine Benkemoun Jun 11 '09 at 5:15
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Take a look at these wikipedia articles:

Microsoft has a hypervisor called Hyper-V.

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also look for "bare-metal virtualization" –  Rob Allen Jun 9 '09 at 16:54
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KVM for Linux

While it very much looks like just yet another software to use some virtual machines in reality it effectively turns the Linux kernel into a hypervisor.

The same is true for XEN, the difference is that with XEN you actually boot XEN as the Hypervisor which will start a virtualised instance (dom0) that is used as the management instance.

KVM turns your installation of Linux into a hypervisor so that you essentially the distribution you use around your KVM enabled linux kernel isn't much more than a management interface.

Linux KVM and XEN is to my knowledge the only free (as in libre) bare metal virtualizations available.

Other options have already been named

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Yeah, now please run that without OS - i.e. without Linux. Or starting anything. –  TomTom Jun 16 at 10:17
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Its not possible to run VM's with out Host as it is not possible to run process with out operating system.

In an operating system the kernel isolates different process and schedules them there should be something similar to do the scheduling and isolation in virtual environment this kernel is called Hypervisors.

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Basically, there are two kinds of hypervisors. First there are the hypervisors that run on top of your operating system. These are called Type-2 hypervisors and include VMware Workstation, MS Virtual PC and VirtualBox. Then, there are the hypervisors that are an operating system (I know that is over-simplified). These are called Type-1 hypervisors and include Xen, Linux w/ KVM, VirtualIron, ESX, ESXi and MS Hyper-V.

What comes closest to what you want, is a Type-1 hypervisor. Judging you from your question, you are going to have a hard time with just about everything save for ESXi with its web interface. I'd go for that if you want decent performance and the option of a broad variation of guest operating systems. ESXi is a bit picky for the underlying hardware though, so if that is a problem, look into Fedora w/ KVM and / or Windows 2008 with Hyper-V. Latest option is pretty expensive though, if you do not have a Windows 2008 license already.

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Yeah, but even a bare metal hypervisor IS - technically - an OS. So the answer is wrong. –  TomTom Jun 16 at 10:16
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A virutual machine relies on the host machine, and its OS, to provide access to the host machine's physical CPU, memory, disk drives and other peripherals.

So, no, you cannot run a virtual machine without a host machine and its OS.

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bare-metal hypervisors, anyone? –  HannesFostie Jun 11 '09 at 7:46
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Convict is right. Hypervisor is just a hardened, very little os dedicated to manage VM access to resources. Nervertheless, it is an OS, that will host in ring 0 on physical CPU. Intel VT help to offload this task for hypervisor, but VM can't have real access to rinf 0, else you couldn't have virtual machine isolation between them. –  Mathieu Chateau Jun 11 '09 at 8:18
    
Mathieu Chateau's answer is more complete than Convict's. While convict is technically correct, I think it should mention type 1 hypervisors and why they still don't provide direct physical (ring0) access to the virtual machines. –  Burly Feb 18 '10 at 15:59
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You can use Windows 7 and boot directly to a virtual hard drive, then the OS on that virtual drive will have direct access to hardware. Check out this article: Less Virtual, More Machine - Windows 7 and the magic of Boot to VHD

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Very interesting. Thanks for posting this. –  TwistedAphid Jun 9 '09 at 19:00
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That's only a virtualized disk, not a virtual machine. And you can only run a single OS at once, doesn't seem like much use except for testing Win7 without a spare drive. –  saschabeaumont Jun 9 '09 at 21:55
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I think you're looking for something like VMware ESXi. It's also free nowadays...

Shamelessly copied from this more or less related question ;-)

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You can also look into Citrix XenServer 5.0

It's totally free since february and it's got really good performance.

The other (free and easy to manage) options earlier described are

  • VMware ESXi
  • MS Hyper-V is free if you allready have a Win2k8 license
  • Novell Xen (comes with Novell SLES)

Don't forget that when you are using a Bare-metal hypervisor (as the ones in the list), you'll need an Intel VT or an AMD-V enabled CPU for running Hardware Virtualised Virtual Machines. With VMware and Hyper-V, you have the opportunity to run Binary Translation Virtual Machines which has a more negative effect on performance than hardware virtualised.

Novell Xen and Citrix XenServer are built upon Xen (duh) so it's really easy to set up paravirtualised virtual machines. This kind of virtualization requires a kernel that is aware that it is being virtualized. Novell SLES is really easy to set up paravirtualized and Citrix XenServer has a Debian Paravirtualised template on board that immediatly installs Debian without the need for a CD. february 18, a new version of XenServer will come out, including Debian Lenny (the latest Debian).

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