Beginning with the disk tests, where the effects of KVM are very noticeable, we had numbers from AIO-Stress. When measuring the random write performance, the host OS was able to write at 31.31MB/s, which is no comparison to 1.75MB/s when running Ubuntu 9.10 within a KVM-driven virtual machine.
With IOzone we looked more closely at the disk performance for both reads and writes using a 4GB file-size. The read performance ended up not being as nearly impacted as the write performance, but there still was a performance hit obviously.
The disk transaction performance with PostMark had also dropped by more than half under the virtual machine that was running as a single file on the host OS.