Currently for XenServer, Acronis will only be able to backup via the Agent in Guest OS method. This applies for both Acronis Backup Cloud and Acronis Backup Advanced. Other backup software is also having this tech limitation apparently.
For the major hypervisors including XenServer, Acronis Backup Cloud will be able to recognize the VM even if you are using Agent in Guest OS. So meaning that the device will be recognized as a VM during billing.
So this is good news as the pricing will remain the same as VM pricing, and you will still be able to perform Full Machine, Disk Volume, File/Folders, Application (Require App Agent installed) on the XenServer VM.