The first disk of a XenServer guest VM is called xvda, the second disk (the one we added) is called xvdb. We start with the creation of a Physical Volume (PV) on the extra disk.
pvcreate /dev/xvdb
Then we extend the existing Volume Group (VG) called VolGroup00.
vgextend VolGroup00 /dev/xvdb
Then we extend the Logical Volume (LV) by the size of the extra disk, in this case 8GB.
lvextend -L8G /dev/VolGroup00/LogVol00
Finally we resize the filesystem that uses this LV.
resize2fs /dev/VolGroup00/LogVol00
Or use this https://pixel.red/guide/xen-online-resize-vm/
