EDIT 8-2-2013: A new update KB 2744129 has superseded the original KB 2526776
Please note that this new update now applies to Hyper-V 2008 and 2008 R2 -- the original was only for R2.
If you are testing out Windows 8 as a VM which is hosted on a Windows 2008 or 2008 R2 Hyper-V server, then you will need to install the following update.
An update that enables the Windows Developer Preview or Windows Server Developer Preview to be hosted in a Hyper-V virtual machine on Windows Server 2008 R2
You cannot run a Windows 8-based or Windows Server 2012-based virtual machine in Windows Server 2008 or in Windows Server 2008 R2
The issue occurs because the Hypervisor does not handle the one-shot synthetic timer that is also known as an aperiodic timer correctly.
Note that this will require a reboot of the host machine, and if you do not install the patch the host may produce a stop screen, and restart which will impact other running VMs on the host.