The SQL team has created a performance guidance document with solid suggestions and performance numbers when running on WS08 Hyper-V. They looked at OLAP and OLTP workloads. Even if you are not running SQL it will give you and idea of how other database like workloads are likely to perform.
This white paper describes a series of test configurations we ran, which represented a variety of possible scenarios involving SQL Server running in Hyper-V. The paper discusses our results and observations, and it also presents our recommendations. Our test results showed that SQL Server 2008 on Hyper-V provides stable performance and scalability. We believe Windows Server 2008 Hyper-V is a solid platform for SQL Server 2008 for the appropriate workload. It is practical to run production workloads under a Hyper-V environment, as long as the workload is within the capacity of your Hyper-V guest virtual machine.
Download (White paper)