The SQL Performance Team has published an excellent document on running SQL Server 2008 in a Hyper-V environment. The document is 37 pages long and has detailed performance information as well as recommendations. The full document can be downloaded at:
The key section (as far as I am concerned) is the conclusion:
From a performance perspective, Hyper-V is a viable option for SQL Server consolidation scenarios. The overall performance of SQL Server running in a Hyper-V virtualized environment is reasonable compared with the equivalent native Windows Server 2008 environment.
With proper I/O capacity and configuration, the I/O overhead is minimal. For best performance, you should have enough physical processors to support number of virtual processors configured on the server to avoid overcommit CPU resources. The CPU overhead increases significantly when the CPU resources are overcommitted. It is important to test each application thoroughly before you deploy it to a Hyper-V environment in production.