A sample of 1
I've actually played with KVM, Vmware and Virtualbox before I built my last system, to see how each worked.
The command line junky in me loved KVM and could live with Virtualbox. I hated vmware on that account, at least on the products I could try then. But in the end, the one thing that impressed me the most about KVM was the number on my UPS that indicated energy usage in Watts.
KVM was easilly the most awesome one on that account. On the same machine, at idle, the machine would consume a consistant 20 watts less energy then with either of the other 2, making it drain 115W instead of 135W with the other 2.
Doing a performance benchmark in the VMs afterward showed the footprint of the virtualisation layer was also very different. KVM would cause around 5% overhead and was extremely consistent, compared to between 8% and 15% with the other 2, with a lot of fluctuation, not always for obvious reasons, or any I could find really.
A sample of 1, so YMMV