RH based grub boot loaders need to have something where the kernel line is a xen.gz and the rest are modules
title CentOS (3.8.8-1.el6xen.x86_64)
root (hd0,0)
kernel /xen.gz cpufreq=xen dom0_vcpus_pin
module /vmlinuz-3.8.8-1.el6xen.x86_64 ro root=/dev/mapper/vg_worf-lv_root rd_NO_LUKS LANG=en_US.UTF-8 rd_LVM_LV=vg_worf/lv_root rd_NO_MD SYSFONT=latarcyrheb-sun16 rd_LVM_LV=vg_worf/lv_swap crashkernel=128M KEYBOARDTYPE=pc KEYTABLE=us rd_NO_DM rhgb quiet
module /initramfs-3.8.8-1.el6xen.x86_64.img