This first one applies to everyone:
2397711 – Recommended hotfixes for System Center Virtual Machine Manager 2008 R2
Hyper-V Update List for Windows Server 2008
Hyper-V Update List for Windows Server 2008 R2
Some other fixes that might be relevant, depending on your setup:
975921 – You may be unable to perform certain disk-related operations after an exception when a hardware provider tries to create a snapshot in Windows Server 2008 R2 or in Windows 7
975354 – A Hyper-V update rollup package is available for a computer that is running Windows Server 2008 R2
975530 – Stop error message on a Windows Server 2008 R2-based computer that has the Hyper-V role installed and that uses one or more Intel CPUs that are code-named Nehalem: "0x00000101 – CLOCK_WATCHDOG_TIMEOUT"