I'm interested in virtualization and have some questions about type of hypervisors and their implementation.

  • What I know is that we started with type 2 hypervisors. These get installed on the operating system just like any other application. This makes sense to me, the hypervisor is just an application.

  • What I don't understand is type 1 hypervisors, like vmware ESXi. How exactly is it possible to run this on the bare metal? Does it mean that this hypervisor is an operating system in itself? How is this sort of hypervisor implemented?

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    This is all marketing... all what manages hw and provides userland is for me OS. – jirib Mar 16 '12 at 8:00
  • Where is all this talk about Type 1 and Type 2 hypervisors coming from? I've never heard them called that before, and now this is twice in two weeks. – Mark Henderson Mar 16 '12 at 17:56
  • @MarkHenderson it's really a marketing/edu thing, see virtualizationreview.com/blogs/everyday-virtualization/2009/06/… as an example of the entire "who made this crap up anyway" That being said it's pretty useful to explain some of the different types of virtualization available. – Jim B Mar 17 '12 at 1:26

VMWare ESX(i), as mentioned, is a Type-1, bare metal hypervisor. This means, yes, ESX(i) is a very lightweight operating system that can be installed to a USB drive or SD card in the server, leaving more local storage for the virtual machines. I don't know how it is implemented or works, though.

VirtualBox and Virtual PC are both Type-2, while Hyper-V is avalible in both Type-1 and Type-2 roles.

VMWare is NOT an operating system - they are the company that develop the ESX/ESXi/vSphere/vCentre Server packages.

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    This is accurate, ESXi is actually a full fledged OS, its just hidden away to where you cant normally see it. If you enable the rescue console you can log into the box and get a normal linux console. – Patrick Mar 16 '12 at 8:58
  • @Patrick, so it's just linux? – sameold Mar 16 '12 at 9:03
  • @sameold its not pure linux, as they have taken the kernel and make some modifications to it, but basically its linux. – Patrick Mar 16 '12 at 9:16
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    At one point in the distant past, ESX had some Linux bits in it. That is not the case anymore, though. The ESXi kernel is 100% custom. – EEAA Mar 16 '12 at 12:19
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    hyper-v is not available in a type 2 role. It's simply available without windows. – Jim B Mar 17 '12 at 1:25

It's not linux and, if you believe VMWare, it is not based on linux.

VMWare has repeatedly stated that the vmkernel that ESX/ESXi boots into is the hypervisor and is not derived from linux.

A customized linux build (based on RedHat) is loaded as the first virtual machine running on the vmkernel and is used for management of the vmkernel itself.


The real question is "what's an OS"? In a basic sense an OS is a piece of software that starts at machine boot and provides a set of services for developers to use to write applications. In that sense VMware (or any hypervisor) is an OS.

What makes a hypervisor different is that a hypervisor treats other operating systems as applications and intercepts calls to the underlying hardware. The service that a hypervisor OS provides is virtual hardware to run other OSes (including in some cases other hypervisors). The hypervisor manages shared access to the underlying physical hardware. Hypervisors can run "applications" but the applications are usually things that extend or replace the virtual hardware (Ciscos nexus 1000v on vmware and hyper-v is an example of a software networking application that replaces the built in virtual networking environment)

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