with FC5 2.5.16.1-2133 and xen 3.2

to get zen to boot out of grub I had to turn off the xend on startup with chkconfig and
then Zen would come up into X Windows. You then have to setenforce=0 and then start
xend.  

chkconfig --level 345 xend off
....
boot xen into xwindows
.....
setenforce=o
xend start
...
now xen is ready for a guest.

Ron Wahler
http://www.positive-logic.net