QLogic 2600 Series Hyper-V Virtual Server Engine - Manual (Page 9)

The size of a dynamically expanding VHD is as large as the data that is written to it. As more data is written to a dynamically
expanding VHD, the file increases to a maximum size. A differencing VHD is similar to a dynamically expanding VHD, but it contains
only the modified disk blocks of the associated parent VHD. Dynamically expanding VHDs are useful for testing environments
because there is less impact if you have to rebuild the VHD. For example, some of the tests performed for this report used multiple
dynamically expanding VHDs, each with a different Windows image. Fixed VHDs are recommended for production.
New Hyper-V Performance
VHDX
A new Hyper-V VHD format introduced with Windows Server 2012 that
increases storage capacity from 2TB to 64TB.
25 Percent More Throughput with Windows Server 2012 Hyper-V
One of the most important features in Microsoft Windows 2012 Hyper-V for I/O performance is the VHDX
virtual hard disk format, which provides storage for the guest OS. Testing by Microsoft shows that VHDX
delivers nearly 25 percent better write throughput than virtual hard disk (VHD) for both dynamically
expanding and differencing disks.
VHDX Performance—1MB Sequential Writes