Only works if Ubuntu is setup to use LVM!
Steps to follow:
Stop the running VM from within the VM.
Go into Xen Orchestra and increase disk size of the running VM.
Start the VM again.
Check partitions: sudo lsblk
Grow the relevant partition: sudo growpart /dev/xvda 1
Check again: sudo lsblk
fdisk -l
pvresize /dev/xvda1
lvdisplay
lvextend -l +100%FREE /dev/ubuntu-vg/ubuntu-lv
resize2fs /dev/ubuntu-vg/ubuntu-lv
final check df -h
Useful links:
https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/recognize-expanded-volume-linux.html
https://carleton.ca/scs/2019/extend-lvm-disk-space/