Virtualization-related overhead can’t be boiled down in to a single number or percentage. Rather, it varies significantly with the specific resource usage needs of a VM and the applications it’s supported. Administrators should focus on the end-user experience which measuring performance. This often involves simulating transactions or common operations within an application. Most initial reports show that Hyper-V significantly outperforms Microsoft Virtual Server, especially for supported guest OSes. There are numerous ways to monitor and troubleshoot performance. One approach is to use tools like the built-in Performance Monitor in Windows Server 2008 or from within guest OSes. You can also use Microsoft System Center Virtual Machine Manager (SCVMM) to monitor in larger environments. Overall, with the right tools, you should be able to measure the performance of your virtual workloads and make adjustments wherever necessary.