After initial installation of Citrix Xen Server, you are supposed to add ISO repository for installation of VM. Below are the steps to do so.
# mkdir /home/iso # xe sr-create name-label=ISO-Repo type=iso device-config:location=/home/iso device-config:legacy_mode=true content-type=iso
After putting this commands in CLI mode you will be getting one device ID. Alternatively, you can check the same in Xen Center.