Moving VM’s in XenServer

It'll use either LVM or files-as-block-devices. Either are trivial to copy, whether or not you can move it to an external drive depends on what hardware you've got, and what you want to achieve.

Existing fully-virtualised VMs from other systems can be "imported", in the sense that what was previously running on another system can be made to run in Xen. It may require significant jiggling to make that work, though, depending on what you're moving from.

Were you guided through the network setup before this screen appeared? Either the virtual network card isn't being detected, or more likely, Xen isn't running DHCP so you have to manually configure the IP/DNS/Gateway settings before you can complete the netinstall.