Status

KVM is included in the mainline linux kernel since 2.6.20 and is stable and fast for most workloads.

It is also available as a patch for recent Linux kernel versions and as an external module that can be used with your favorite distro- provided kernel going back up to 2.6.16, therefore including all latest versions for Enterprise Linux Distributions.

Working:

Intel-based hosts (requires VT capable processors)
AMD-based hosts (requires SVM capable processors)
Windows/Linux/Unix guests (32-bit and 64-bit)
SMP hosts
SMP guests (as of kvm-61, max 16 cpu supported)
Live Migration of guests from one host to another (32-bit and 64-bit)
See the Guest Support Status page for a list of guest operating systems known to work
See the Host Support Status page for information on host hardware.
Guest swapping
Paravirtualized networking
Paravirtualized block device

In progress:

PowerPC port
IA64 port
xenner (http://kraxel.fedorapeople.org/xenner), a project to run x86 xen guest (domU) kernels
ARM port