I have not used Novell Xen or Starwind, I have worked with XenServer as well as the Xen packaged with both Debian and RHEL5. I did not try LVMoiSCSI when I tested XenServer as I didn't have an iSCSI host available at the time. That said from my understanding of iSCSI and LVM I can definitely hazard an educated guess which may help you in isolating the problem.
In my experience iSCSI has been an "one system can mount at a time" unless using a cluster aware filesystem like GFS. In my own Xen servers I use LVM to slice off the RAID array to be used as physical devices by my domUs. I am assuming Citrix has worked this into their LVMoiCSCSI support to do the same thing with an iSCSI volume. I will definitely have to try this out and see if I can't confirm my suspicions.
If Citrix's LVMoiSCSI doesn't do anything special other than treat the iSCSI LUN as a logical volume (LV) (ie- nothing to make it cluster aware) you could attempt to have your Novell Xen systems to do the same thing. LVM is by default cluster aware so each LV created would be able to be separately mounted by different servers while they all have the iSCSI target LUN made accessible.
LVM itself is fairly easy to setup and work with and the commands should be straight forward across any Linux distribution.
The first thing you would want to accomplish, which I'm assuming you've already done, is make sure the server can access the iSCSI LUN and see it as a local SCSI drive.
Once you're able to see the iSCSI drive from at least one of the systems you would want to initialize it as a Physical Volume (PV):
Of course replace the device with whatever device your system sees the iSCSI LUN as. In my experience this changes from reboot to reboot sometimes.
With your PV initialized it's time to create a Volume Group (VG) and tell it to use your initialized PV:
vgcreate XenVG /dev/sdX
Where XenVG will be your VG name and using the same device you used in the PV initialization step. Now if you run
vgdisplay you should see something like:
--- Volume group ---
VG Name XenVG
Metadata Areas 1
Metadata Sequence No 9
VG Access read/write
VG Status resizable
MAX LV 0
Cur LV 1
Open LV 1
Max PV 0
Cur PV 1
Act PV 1
VG Size 204.72 GB
PE Size 32.00 MB
Total PE 6551
Alloc PE / Size 640 / 20.00 GB
Free PE / Size 5911 / 184.72 GB
VG UUID tMHTWV-1dYR-4yB1-tmS5-q1Tk-i3Yx-6l1YLa
This was taken from one of my live Xen servers with a single 20GB LV slice already setup. From this point it's just simply a matter of creating LVs for your domU drives. In the simplest form you can do so as:
lvcreate -L <size> -n <LV name> XenVG
Setting to the desired drive capacity for the domU and I typically set equal to the hostname of the domU I'm creating it for.
Then when you go to install your domU you would specify the disk as
/dev/XenVG/<LV name> and Xen will treat it as a physical device.
This should allow you to have the LVs be mounted by different Xen servers from the same iSCSI LUN. You couldn't have the same LV mounted and used on two machines simultaneously; however, if you had some form of HA setup you could have the nodes start/stop the domU on different machines to maintain the virtual servers availability.
There are also many more options to the above LVM commands, I merely gave the simplest forms. I would highly recommend reading the man pages and there are several good LVM HowTos available online as well.