Basic steps are as follows.

1. Shutdown VM
2. From XenCenter, insert the CentOS iso into the VM’s Virtual DVD drive.
3. Boot the CentOS VM in recovery mode.  If you need help with that check this post out.
4. On the grub menu, select recover OS Installation.
5. Run through the recovery and mount the VM’s disk where CentOS is installed
6. You should now be able to drop to a prompt and chroot /sysimage
7. Change the root password with passwd
8. Shutdown the VM
9. Eject the CentOS iso
10. Boot up the VM and login with the new password