Unlike VMWare (ESX/ESXi), Citrix’ XenServer presents virtual hard drives over an IDE interface which is limited to 4 devices (two primary, and two slave devices), and since you also have an optical media (DVD) present, then you are limited to only 3 physical drives presented to your virtual host within Xen.
However, according to the documentation, the maximum number of disks which can be added to a Microsoft Windows VM is 7 physical drives. However, without reading the details on how to accomplish this, you will find that you can actually only add 3 physical drives from XenCenter.
To increase this, you will need to first boot the Virtual Server and install the Xen Tools, and reboot so that XenCenter detects the correct toolset is installed. At this point you will be permitted to add a second set of 4 physical drives, which will be available within Disk Management immediately. Of course you will need to go through the customary partitioning and formatting of the virtual drive.
You need to be aware of this issue as booting a migrated system without all the necessary drives attached MAY cause issues. In some cases it COULD even prevent the server from booting.