Edit grub.conf
nano /etc/grub.conf
Change the "default=x" to the xen kernel. Example below, the default=0 mean the first title in the hiddenmenu
default=0
timeout=5
splashimage=(hd0,0)/grub/splash.xpm.gz
hiddenmenu
title CentOS (2.6.18-92.1.6.el5xen)
root (hd0,0)
Then reboot vps. Sometime may need to fixcentos5xen.php