Logical Volume Manager
The Logical Volume Manager or LVM for short is the typical Linux technology used to carve block devices into multiple logical volumes. LVM also allows some advanced features like snapshots and encrypted disks. Not to be confused with the disk snapshots that BitFolk can provide for you.
LVM at BitFolk
BitFolk VPSes are not initially set up with LVM, because most customers do not ever need more than a single root filesystem, which they never resize.
Even if they do later grow this filesystem, doing so with a single filesystem is trivial and doesn't justify an LVM configuration.
However, for those with more complicated needs there is no reason why LVM should not be used. When there's a need for several separate filesystems and/or frequent resizing, LVM will prove much easier than trying to add, remove and resize Xen virtual disk devices.
Most modern Linux distributions come installed with LVM kernel and userland support, so doing a self-serve net install is probably the easiest way to get a VPS which is running entirely from LVM. If you want your operating system (i.e., root filesystem) to remain running from the usual Xen block device whilst using LVM for everything else then this is also easy and you wouldn't need to reinstall. You'd just buy some more disk space and specify that you want it as a new disk, and then use that disk (e.g. /dev/xvdc) as an LVM physical volume.