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xenserver 5.5 update2 running on Pentium-D without VT. says it cant support Windows guests(HVM not found etc) on the same CPU ESXi 4 installed and runs Win XP fine. any thoughts? thanks in adv.

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Windows doesn't have a Xen kernel modules available. Xen can only do paravirtualization without VT. To run Windows Xen needs VT to run full hardware based virtualization. I don't know exactly how ESX does it though.

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"Xen can only do paravirtualization without VT" i think you mean with VT? – John-ZFS Apr 2 '10 at 14:05
No he is correct. For Xen without VT hardware support the only option is paravirtualization which means the guest must be modified to run. – Helvick Apr 2 '10 at 15:01

ESXi support Windows guests by virtualizing the whole of it rather than using virtualization extensions, basically. VirtualBox will do the same. Neither VMware nor VirtualBox will create a 64 bit guest without VT-x/SVM, however.

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ESX can handle this because it can use binary translation to run unmodified Windows Guest Operating Systems while Xen does not have this capability.

VMware's binary translation technique was their original approach to virtualization as their products predate hardware x86 virtualization by a number of years and they have always supported running unmodified Guest Operating Systems. Basically a VMware hypervisor can intercept potentially harmful Guest instructions on the fly and modify them. Xen (and Hyper-V and most others) do not use such techniques, their options are either paravirtualization or hardware virtualization. There's a nice explanation of how vmware's binary translation works in this Anandtech article.

Paravirtualization requires that the Guest OS is modified in advance so that the potentially harmful instructions\operations are replaced by code that will run safely in a Virtual Machine. The term is sometimes referred to as sing enlightened Guests or drivers and it is used by all the major virtualization products to some degree - VMware support a paravirtualized SCSI driver in their latest releases for example and Hyper-V's enlightened driver's are required to run VM's at optimal performance levels.

Hardware Virtualization enables the CPU to handle the instructions that would otherwise have to be dealt with by binary translation or paravirtualization and it allows a Hypervisor to run unmodified guests. While there have been some hiccups along the way the current hardware virtualization implementations allow (most) Guest OS's to run at near bare metal speeds.

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does xen run Linux guest without VT? – John-ZFS Apr 3 '10 at 17:26
Yes - integrated as of Kernel 2.6.23, patches for other versions are available - full list of Guest's that can be run via paravirtualization is in the Wiki article - – Helvick Apr 3 '10 at 18:23

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