Bottom line: Use SCSI virtual drivers with Virtual Server, use IDE virtual drivers with Windows Server Virtualization (WSv)

Very informative post here.

The most interesting for WSv/Viridian/Hypervisor in Windows 2008, is the fact that the IDE virtual driver is "better" than the SCSI counterpart, which is the opposite of what the case was with Virtual Server.

The part of the blog post relating to WSv's IDE driver:

The IDE controller remains an emulated device, but with a couple of differences to the IDE controller in Virtual Server. It is now 48-bit LBA capable . This allows you to connect large VHDs up to 2040GB to it. The second difference is a filter driver we insert into the storage stack inside the guest which effectively bypasses the emulation path for IDE, making it much higher performance. In fact, for I/O paths, the IDE controller with the filter driver performs equivalently to the SCSI controller in Windows Server virtualization. You can also attach pass-through disk storage to IDE, which was not possible in Virtual Server.