Last week I began in running a fresh set of tests using Fedora 13 on a dual AMD Opteron "Shanghai" workstation and had benchmarked the host along with an x86_64 KVM installation facilitated by Red Hat's virt-manager. That all went well, but some problems with the current release of Oracle's VM VirtualBox on Fedora 13 postponed the testing for now. However, if you are interested in seeing how KVM compares to the host performance (when the guest VM was allowed to tap all eight of the CPU cores and the maximum amount of system memory), the current results can be found on Phoronix Global. This virtualization testing was done with the stock F13 kernel, which is based upon Linux 2.6.33.
Hopefully in the near future this testing will be complete. Being thrown into the testing mix too will hopefully be VMware's virtualization platform. If you have any test requests, be sure to let us know within the forums.