You may be prompted for Red Hat Enterprise Linux Installation CD-ROMs during the initial installation to add additional software components or packages. This method can make any ISO, or file based image, available as a device for your guest. This might be useful for installing software from an ISO file or other media source(for example, a downloaded image).

The syntax for adding an ISO image as a new device to a guest is as following:

['file:/var/lib/xen/images/win2003sp1.dsk,hda,w',\
'file:/xen/trees/ISO/WIN/en_windows_server_2003_with_sp1_standard.iso,hdc:cdrom,r', ]

In the example above you can see how the file en_windows_server_2003_with_sp1_standard.iso has been added to the guest configuration. Once you reboot your guest this ISO image will be configured as a CD Rom driver inside guest.