Windows Server 2008 R2 includes Hyper-V R2 and is available only in x64 edition. I moved my primary lab machine to WS2008 R2 and is really benefitting from these features

• I can create VHDs and mount them as drive letters using “Disk Management”. So I can add/remove files to VHDs without using virtual machines
• VHD files can now be added to virtual machines while they are running. So no need to shut down virtual machines if I want add more storage to it.
• Though I’m not using it very often “Live Migration” is the killer feature. This is a fitting reply to VMWare “vmotion” feature. It comes “free” with Windows server license. More importantly it works on almost any hardware and takes less than 15 minutes to configure.
• The “processor compatibility” feature is allowing me to perform “live migration” between computers with different processor numbers