PAE confusion using FC5 kernel-xen - is it PAE or not?

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Hi,

I've been playing around with FC5 and Xen, using kernel-xen0 and 
kernel-xenU for dom0 and domU's respectively..
Unfortunately I hit a snag in that I can't get pci pass-through for 
ethernet cards to a xenU kernel as there's no support for ethernet card 
drivers.. so I had a look at the new kernel-xen. 

And I've hit a confusing snag, to do with PAE where I get conflicting 
messages in xend-debug.log.
Two scenarios give different PAE messages:

Scenario 1:
dom0:  kernel-xen-2.6.17-1.2174
domU: kernel-xen-2.6.17-1.2174
This gives the xend-debug.log messages:
ERROR: Non PAE-kernel on PAE host.
ERROR: Error constructing guest OS

Which I thought was odd, as they're the same kernel?

So I tried switching dom0 back to a non-PAE kernel build.. xen0

Scenario 2:
dom0: kernel-xen0-2.6.17-1.2174
domU: kernel-xen-2.6.17-1.2174
However, when I try to "xm create" the domU this gives me:
ERROR: PAE-kernel on non-PAE host.
ERROR: Error constructing guest OS

So my confusion is that I'm being told that kernel-xen is a PAE kernel 
by the debug, *and* that its a non-PAE kernel by the debug.  I'd have 
thought that scenario 1 above would work.
I checked the files in kernel-xenU and kernel-xen with an "rpm -ql", and 
can't see any conflicts in /boot - they have separate
filenames for config, initrd, system.map, vmlinuz kernel etc.

Anyone have any idea whats up here, and how I can get kernel-xen to boot 
on the DomU so I can get the pass-through working? (I want to set up one 
of the domU's as a gateway/proxy/firewall for the others)  I can't use 
FC6 currently - it has no support for combined software RAID5/LVM2 
setups which is what I'm running for dom0.
My grub.conf on the domU is:

#boot=/dev/xvda
default=1
timeout=5
splashimage=(hd0,0)/grub/splash.xpm.gz
hiddenmenu
title Fedora Core (2.6.17-1.2174_FC5xen)
       root (hd0,0)
       kernel /vmlinuz-2.6.17-1.2174_FC5xen ro root=LABEL=/ rhgb quiet
       initrd /initrd-2.6.17-1.2174_FC5xen.img
title Fedora Core (2.6.17-1.2174_FC5xenU)
       root (hd0,0)
       kernel /vmlinuz-2.6.17-1.2174_FC5xenU ro root=LABEL=/ rhgb quiet
       initrd /initrd-2.6.17-1.2174_FC5xenU.img
title Fedora Core (2.6.15-1.2054_FC5xenU)
       root (hd0,0)
       kernel /vmlinuz-2.6.15-1.2054_FC5xenU ro root=LABEL=/ rhgb quiet
       initrd /initrd-2.6.15-1.2054_FC5xenU.img

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