I just tried running my Windows XP KVM on fedora 13
(I installed the KVM under fedora 12). It seemed to work
well, but I'm using the redhat virtio drivers for
network and disk, and Windows insisted on re-installing
the drivers (claiming "new hardware") the first time
I booted under fedora 13 libvirtd.
Strangely, when I booted the same KVM after going back
to fedora 12 on the host, it didn't insist on
re-installing the drivers yet again.
None of this triggered any Windows activation nonsense
(which is why it is only annoying :-).