boot Xen From grub
 

I wanna know how to boot a xen from grub2 ? and is there a way to boot a domU also from the grub (if it is installed in a disk image ex :- disk.img)

To boot xen from grub, you have to add xenified linux kernel to the grub.conf. you have already done so and facing any problems, i suggest you post your grub.conf file.

You cannot boot domU from Hosts grub as it requires a Dom0 to be running first. However, you have have a startup script which can start the domU as soon as Dom0 starts.