Virtualization Review published a comparative performance test of three hypervisors: VMware ESX 3.5, Windows Server 2008 Hyper-V and Citrix XenServer.

The results?
Windows Server 2008 Hyper-V won 4 of the 11 tests, XenServer won the other 7 by a narrow margin, and ESX won 0.

For a good commentary on the results, and a bit of easy-read analysis, see the Microsoft Hyper-V blog here