Daveberm on Clustering For Mere Mortals had did some good effort to test new Dynamic VHD files in windows 2008 R2, With the release of Windows Server 2008 R2, one of the enhancements was improving the performance of dynamic VHD files. Prior to R2, writes to dynamically expanding VHD files could be 3x slower than writes to a fixed size VHD file due to limited meta data caching. Overall, Microsoft is claiming the performance of dynamic VHD files vs. fixed size VHD files is almost identical.

Pass-through disks are another option when configuring a Hyper-V VM. According to my results, the performance of a pass-through disk is marginally better than that of VHD files. However, if you use pass-through disks you lose all of the benefits of VHD files such as portability, snap-shotting and thin provisioning. Considering these trade-offs, using pass-through disks should really only be considered if you require a disk that is greater than 2 TB in size or if your application is I/O bound and you really could benefit from another .1 ms shaved off your average response time.

Check it there