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:
Enter xenserver grub 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.
Edit xenserver grub menu
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:
update xenserver boot menu netry

Boot into single mode

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

Reset your root password

Once you enter into a single mode use passwd command to reset your XenServer's root password:
use passwd command to reset 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
reboot xenserver

New root password

If everyting went well you should now be able to login with your new XenServer password:
xenserver started after reboot
test your new reset xenserver root password