Microsoft posted a documentation on “how to deploy Hyper-V Server 2008 R2 on USB Flash drive”. Even cooler than booting Hyper-V off of a USB device is the fact that what we’re doing here’s booting a VHD with Hyper-V installed on it off of a USB device:

1. Download BootHVSR2FromUSB tool, with this tool all you’ve to do is:
2. Install Windows Automated Installation Kit (WAIK)
3. Grab Microsoft Hyper-V Server 2008 R2 and either burn it to a DVD or mount it with a virtual CD program
4. Connect your USB flash device
5. Run tool (step 1) As Administrator
6. Select your USB flash device as your target disk
   1. Warning! You can select any disk in your system and it’ll get formatted. Make sure to select your USB flash disk, and you don’t have any data on the disk.
7. Click on Create Blank VHD button and choose a temporary location for VHD
8. Locate your copy of INSTALL.WIM file from Microsoft Hyper-V Server 2008 R2
9. Hit Start button and wait a short time (~30 minutes on my system) for everything to be setup
10. When it is finished you can now take your USB flash disk and boot a system off of it. Once you’ve completed standard Hyper-V Server post installation configuration – you can connect to system remotely and start partying with Hyper-V.

More info: TechNet