I'm using KVM now on a rather beefy OpenSUSE system. Since I'm still fiddling with NAS backup and UPS scripts, I cannot tell you what works and what doesn't, but if you're going for KVM, you should consider using non-cirrus video card since it failed on the AV Wizard after reboot (X couldn't find a monitor), and it didn't help changing it after install (for some reason). So choose VGA on the vm setup. I'm still searching for ways to use the nVidia card on the virtual guest.
Also, I had to add the 2nd network card to the guest OS AFTER install..
I'm a long time Virtualbox user, and so far I am really impressed with KVM both in terms of performance and ease-of-use. Please check out the nice OpenSUSE docs: http://doc.opensuse.org/documentation/html/openSUSE/opensuse-kvm/
The Yast control panel in OpenSUSE makes it a whole lot easier and polished:)