VHD (short for Virtual Hard Disk) is a file type spec that allows a hard drive system to be contained into an individual file. This file can be used as a boot partition on physical hosts and virtual machines. The VHD format is NOT proprietary to Citrix but is rather a Microsoft-maintained technology. We use it in PVS as a virtual disk image of an Operating System that a PVS target device can boot to from the network. For more information on VHD, visit this MSDN web resource.