If you load a Windows Server 2008 R2 virtual machine on a Windows Server 2008 Hyper-V host server, you will get an error on the host server similar to the following:
Log Name: Microsoft-Windows-Hyper-V-Worker-AdminThis happens because the Windows Server 2008 R2 Hyper-V Integration Components are not compatible with the Hyper-V v1 release components.
Date: 10/23/2009 7:56:48 AM
Event ID: 23012
Task Category: None
User: NETWORK SERVICE
Device 'VMBus' in 'EX1 ENT x64' cannot load because it is incompatible with virtualization stack. Server version 13 Client version 65537 (Virtual machine 98EEEED7-A97D-48CF-87F5-E1E8F698D169).
Incompatible does not mean they won't work - because they do. It's just that the R2 version includes enhancements and changes that are beyond the capabilities of Hyper-V v1.
If you want to run an R2 build in a VM on Hyper-V v1 and you don't want to see this error, use a Legacy NIC for the R2 VM.
The Integration Components are already present in Windows Server 2008 and Windows Server 2008 R2. You do not need to install them on these VMs. You can only upgrade the Integration Components, not downgrade them.