"Why should VMs be mounted on physical machines?
Xen uses a virtual hard drive (VHD). This virtual hard drive is stored somewhere on the Xen-based host machine. For the purposes of this tip, I will refer to this "somewhere" as the storage back end. There are two common ways to configure the storage back end. The easiest solution is to use a disk image file which is created in the host operating system. This also is the location where the VM installs its files. The alternative to this method is to use a device as the storage back end. This can be a logical volume, a partition or a logical unit number (LUN) on the storage area network (SAN). Essentially you can use any storage device for this purpose, and due to the performance benefits this solution is used most often.
"By mounting a virtual machine in the host operating system (OS), you can do all maintenance that you need to do rather easily. If the virtual machine runs Linux as its operating system, this is particularly easy because the Xen host OS is Linux as well. It also helps that the most useful tools for VM maintenance in a Xen-based environment are installed by default in the Linux host. But even if the virtual machine uses Windows there are lots of options. You use one of the many new technology file system (NTFS) maintenance tools that are available nowadays."
