How do virtual disk files differ among the hypervisors?
VMware uses a proprietary Virtual Machine Disk (VMDK) format that it developed, and XenServer and Hyper-V use a proprietary Virtual Hard Disk (VHD) format. These formats are completely different from each other and cannot be used directly with another hypervisor. When performing a virtual machine migration, there are a number of tools and utilities that will convert virtual disks from one hypervisor format to another.