Update for Windows Server 2008 x64 Edition (KB949219)
 

The Release Candidate 0 (RC0) update to the Hyper-V role provides improvements to security, stability, performance,
user experience, forward compatibility of configurations, and the programming model.
All users of the Hyper-V role are encouraged to apply this upgrade. After you install this item, you may have to restart your computer.

More info can be found here.