Ronald Beekelaar, Microsoft MVP of Virtual Machine Technology has a list of the top-5 things about Hyper-V.
5) Understand the hypervisor model and performance consequences.
This is especially a big one for people that know Virtual PC and Virtual Server. The virtualization model that Hyper-V uses is very different from the model that Virtual PC and Virtual Server use. The Hyper-V model allows for much better I/O performance of virtual machines. This is mainly due to the new 64-bit hypervisor layer underneath everything - including the host operating system even, and the new high-speed VMBus "synthetic" drivers that run in the virtual machines.
Particularly- the use of optimized synthetic disk and network drivers talking to the VMBus, instead of using normal "hardware-oriented" drivers, make for a much faster I/O path from applications inside the Hyper-V virtual machines to the physical hardware. To make use of these synthetic drivers, make sure you use an operating system inside the virtual machines for which Microsoft provides so-called Integration Components. When you install the Integration Components, the synthetic drivers are installed as well.
Please see Microsoft Knowledge Base article 954958 for the list of operating systems in which you can install Integration Components.