The following guide can be used to reset an administrative root password on XenServer 7 Linux.
Enter XenServer Boot Menu
In the first step, reboot your XenServer into Grub boot menu:

Edit XenServer boot menu entry
Use arrows to locate an appropriate XenServer boot menu entry and press e
. Doing so you will edditing mode.

While in the editing mode locate read-only parameter
ro
and replace it with rw
. Furthermore, locate keyword splash
and replace it with init=/bin/bash
. The resulting edited menu item should look similar to the one illustrated below:

Boot into single mode
Once you changed the boot meny entry, hit F10
thus you will boot into a single mode:

Reset your root password
Once you enter into a single mode use passwd
command to reset your XenServer’s root password:

Reboot XenServer
At this stage our XenServer root password should be updated. Reboot your system be entering the command exec /usr/sbin/init

New root password
If everyting went well you should now be able to login with your new XenServer password:

