First: Make sure to shut down the virtual machine or to unmount the disk inside it. Once finished you can start to extend the logical volume on the hypervisor:

lvextend -L+400G /dev/vg0/disk
e2fsck -f /dev/vg0/disk
resize2fs /dev/vg0/disk

There might be a chance you need kpartx, growpart (provided by cloud-guest-utils) followed by resize2fs to grow the mounted partition.

In case you shut down the virtual machine you may start it again with

xm create -c /path/to/config.cfg

which will not only start the virtual machine but also attach you to its console. If you have verified that the disk is now as large as you wanted it to be you can detach from the console with

CTRL + ]