I use Xen with PCI passthrough / win7 guest for 1 year and it works very well.
I decided to check if I can do the same with KVM.
kvm is really easy to install, and I was able to start my win7 VM with virtual machine manager and PCI passthrough with my radeon HD7950 in less than 1 hour.
however the performances are really disappointing compared to Xen.
when I play Chivalry (UT3 based), I have 60FPS with Xen, and it drops to 15-20 with KVM. even when I lower the graphic settings to minimum, 800x600, I always have 15-20FPS. Also I notice that when I have teamspeak running, the VM has 1 CPU in interrupt for more than 50%, and the sound is choppy. (I have a USB sound card, and I use PCI passthrough on the USB controller). I use virtio for network and disk drive, and I spent some hours to have more than 2 CPU in my VM ...
so for me KVM is good, and easy to install, but it lacks performances with windows guest.