In order to use a DRBD resource as the virtual block device, you must add a line like the following to your Xen domU configuration:

disk = [ 'drbd:resource,xvda,w' ]

This example configuration makes the DRBD resource named resource available to the domU as /dev/xvda in read/write mode (w).

Of course, you may use multiple DRBD resources with a single domU. In that case, simply add more entries like the one provided in the example to the disk option, separated by commas.