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Is it still XEN? Or is it VirtualBox, KVM, VmWare or else?

With fastest I mean that the guest VM is fast (the smallest speed loss because of Virtualization)

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The answer will depend mostly on the guest OS and then how you measure fast. Number crunching CPU intensive operations? Video intensive? Access to hardware resources like USB? – Caleb May 28 '11 at 11:30
~~All of them, does a comparing table exists? – LanceBaynes Jun 6 '11 at 19:16
@LanceBaynes: Bad car analogy ahead - "I need the best car." "For what? Racing? Cargo transport? Passenger luxury? Good mileage?" "All of them." – Piskvor Oct 26 '11 at 8:01
up vote 5 down vote accepted

The fastest solution is generally the one that introduce the less overhead compared to a non virtualized environment. If you can cope with its "non OS diversity" limitation, that would be an OS level virtualization implementation. With Linux, that translates to OpenVZ/Virtuozzo, Linux containers (lxc) and VServer.

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Based on what information do you conclude that OpenVZ/Virtuozzo and VServer are the fastest under Linux? – Matt H Feb 13 '12 at 18:36
No specific information but common sense. To clarify, I'm not writing OS level virtualization is necessarily the best solution and has no drawbacks but just it can hardly be beaten as far as pure performance is concerned. – jlliagre Feb 13 '12 at 20:23

Fastest under what conditions? With hardware virtualization, the speed should be identical on all virtualization platforms.

Therefore the only thing that you should consider looking for is hardware virtualization support in the software.

As far as I know, Virtualbox doesn't support IOMMU hardware virtualization yet. KVM, VmWare and Xen should. Xen and VmWare should be the only ones supporting IOMMU on graphic cards (with differing degrees of success).

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Speed is assuredly not identical between all virtualization platforms; the hypervisor is a lot more than a wrapper around the instructions already provided by the CPU. It has to emulate virtual (or paravirtual) hardware, manage memory allocations and paging, schedule CPUs (to my knowledge, nothing but VMware implements relaxed co-scheduling) and so forth. There's lots of benchmarks out there for lots of different workloads on lots of different hypervisors. – jgoldschrafe Oct 26 '11 at 1:38
@jgoldschrafe Nope. When it comes to hardware virtualization the performance should be identical = bellow the measurement error threshold. And emulating hardware is exactly what the hypervisor won't do, when you have hardware virtualization. – Let_Me_Be Oct 26 '11 at 11:38
If you're saying that things like VMware's virtual e1000 network adapter or LSI Logic SCSI controller that are presented to the guest are nothing more than shims to CPU extensions, I have to question whether you've even used a virtualization product before. – jgoldschrafe Oct 26 '11 at 20:01
@jgoldschrafe No, I'm talking about hardware virtualization. When you use para-virtualzation and virtual hardware then yes, it makes sense to compare the performance (in my field it doesn't, since the performance can never match real hardware). – Let_Me_Be Oct 27 '11 at 8:17

I still believe it is XEN. I once had a talk with a RH-pre-sales guy and asked why they kicked out XEN in favour of KVM. He said that KVM is at least as fast as XEN. I asked him to send me proof - nothing came back...

I also disagree with the OS-level. A bare-metal-hypervisor based virtualization has IMHO less overhead than that. So a PV XEN DomU is almost as good as the bare-metal.

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Bare metal or not, hypervisors are an additional layer between the guest OS and the hardware. OS-level virtualization solutions do not need that layer so have much less overhead (if any). – jlliagre Oct 25 '11 at 22:57
The performance difference between OS-layer and full hardware virtualization (with hardware shadow page tables) is negligible; most virtualization platforms these days will have a performance hit under 5% for most workloads. OS-layer virtualization can arguably get you better consolidation ratios, but generally at the cost of QoS and uptime. There's lots of reasons to choose one over the other, but performance is not chief among them. – jgoldschrafe Oct 26 '11 at 1:39
@jiliagre: With OS-level virtualization you also have an additional layer - may it be chroot or some other "hiding" technique. – Nils Oct 26 '11 at 19:33
Sorry for the late reply but you mistyped my nickname. There should be no additional layer with OS-level virtualization. I agree there is some extra code to execute for a small subset of system calls but that should still be negligible compared to what hypervisors have to achieve (i.e. context/mode switches). Not that the latter aren't improving though ... – jlliagre Feb 13 '12 at 21:13
@jlliagre sorry for the typo. The switch is IMHO performed by the CPUs - so the hypervisor has pretty much the same job as with "lightweight" virtualization: scheduling processes and threads. RAM is directly mapped in XEN, too. – Nils Feb 16 '12 at 22:08

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