A. Microsoft provides Hyper-V Server, a free virtualization platform based on the Windows Server Hyper-V role. The latest version is Hyper-V Server 2008 R2, which is based on Windows Server 2008 R2 Enterprise Server Core, so in the free Hyper-V Server, you get clustering capabilities and, therefore, Live Migration.
Normally, you should use Windows Server editions, such as Server 2008 R2 Standard, Enterprise, or Datacenter when hosting Windows Server OSs, because you get virtual OS environment (VOSE) rights with those editions (1, 4, and unlimited respectively). However, in a VDI environment where you're only running client OSs, it's common to want not to buy Enterprise or Datacenter editions because you won't be using any of the VOSE rights. This is where Hyper-V Server comes in—it provides all the functionality of the Hyper-V role running on Server 2008 R2 Enterprise Server Core without any cost.
One common concern is that in a VDI infrastructure, you want to install the Virtualization Host component from Remote Desktop Services (RDS) to let the connection broker stop, start, and communicate with virtual machines. RDS isn't included with the free Hyper-V Server. The good news is that the Virtualization Host component of RDS is part of Hyper-V Server, so Hyper-V Server can act as the machine virtualization layer in your VDI solution with no loss of functionality.