The Performance Tuning Guidelines for Windows Server 2008 has been updated to include perf guidelines for virtualization servers as well as:
Advice in the paper includes:
For some insight into how Microsoft IT uses virtualization (they use SANs for disk storage) listen to the David Lef podcast on How Microsoft IT Uses Hyper-V.
Hyper‑V publishes performance counters to help characterize the behavior of the virtualization server and break out the resource usage. The standard set of tools for viewing performance counters in Windows include Performance Monitor (perfmon.exe) and logman.exe, which can display and log the Hyper‑V performance counters. The names of the relevant counter objects are prefixed with “Hyper‑V.”
You should always measure the CPU usage of the physical system through the Hyper‑V Hypervisor Logical Processor performance counters. The statistics that Task Manager and Performance Monitor report in the root and child partitions do not fully capture the CPU usage.
Here are some recommended links talking about performance tuning and right sizing when planning a deployment