It sounds like a good plan, But i can't help but feel that you have one too many machines in your equation.
And, it may just be how you name them in your head, but i think you are getting JBOD and RAID a little mixed up.
Anyway, the original question.
I'd miss out (X) and just have (W) dishing thing out over NFS. Unless you have (X) and (Y) doing other things, that is.
On another note, You could put all you Virtual machine disks on you "JBOD server" and dish them out by iSCSI. The reason.....
say "Xen server01" has a harddisk or other failure, all you have to do is setup a replacement "Xen server" and point Xen to the iSCSI server and your Windows server are back. It might be a little more complected than i've made it sound.