KVM seems to have a future in centos. I have a couple of servers running kvm, with only 4 cores per server. I tend use 1 real core for each virtual cpu assigned to the guests, because I don't need that many guests. So, I can't speak to scaling... Performance is excellent, however. It's been a year or more since I've tried ESXi or xen on ubuntu, but I was always disappointed in the speed at which the guests ran. That's why I turned to xenserver for its speed and GUI, and then to KVM for its speed and complete control over things like nics and network configurations. KVM works great