You can always grow the storage volume for a running Windows VM in XenCenter, but also there is a way to shrink it. Here is how to do it:
VirtualBox was capable of using physical disks for virtual machines. For more information check out this section on creating raw disks in the VirtualBox manual https://www.virtualbox.org/manual/ch09.html#rawdisk.
This is the scenario that you run Windows as your host operating system and Ubuntu in a VirtualBox, and that you want to access a specific Windows folder from Ubuntu.