Xen vs. KVM

Xen vs. KVM

http://www.peakscale.com/archives/gridvm/xen-vs-kvm/

Dec 20th, 2006 @ 10:15 pm CST

The news of KVM’s inclusion in the Linux kernel
has me looking at KVM.

How the virtualization layer is implemented has important ramifications, and KVM is much different from Xen.

Xen itself sits directly on the hardware and treats even your old OS
as a guest VM — for Xen, the Linux patches are necessary to make Linux a
guest VM on the hypervisor. It implements a model known as
paravirtualization that provides a lot of performance improvements over
other recent x86 virtualization techniques such as VMware’s. This model
includes a service VM that does privileged I/O work on behalf of the
guests. (In a following entry
I discuss some performance and efficiency issues, the larger story is
not so simple as “paravirtualization makes everything very fast”)

Xen can also support unmodified guest VMs if you are using either a VT capable Intel processor or an SVM capable AMD processor.

KVM is a patch to the Linux kernel that is more like VServer
, Solaris containers
,
or microkernels [see footnote], where the OS still sits directly on the
hardware. Some aspects of the current KVM release (mostly pulled from
their FAQ
):

KVM guest OSs appear as a process and can be managed with top, kill, etc.

KVM requires either a VT capable Intel processor or an SVM capable
AMD processor (which is a currently a big limitation for grid providers
but this will become less and less of an issue as older clusters are
phased out).

Uses QEMU for guest instantiation, but is not an emulator itself

Runs Windows 32bit already (with no apci)

I have yet to figure out if it relies on these kernel enhancements for OS level virtualization

The current implementation should probably be seen as a preview, Fraser Campbell reports
that it was “decidedly slow” when he tried it.

[footnote]: Some argue that microkernels and Xen have a lot in common
.

[UPDATE: This article on some KVM details
was posted. Its comments also have a lot of information.]