In this example, the VM name is "debian".

Get the DOM ID of your VM:

[root@xengz ~]# xe vm-list params=dom-id,resident-on name-label=debian
resident-on ( RO)    : 3664c511-cd7b-4705-8fcb-31f8b82d46d5
         dom-id ( RO): 3

If you have multiple XEN servers, you will have to identify which one is running your VM along with the ip address:

[root@xengz ~]# xe host-list uuid=3664c511-cd7b-4705-8fcb-31f8b82d46d5
uuid ( RO)                : 3664c511-cd7b-4705-8fcb-31f8b82d46d5
          name-label ( RW): xen.gz.ro
    name-description ( RW): Default install of XenServer
    
[root@xengz ~]# xe pif-list management=true params=IP host-uuid=3664c511-cd7b-4705-8fcb-31f8b82d46d5
IP ( RO)    : 192.168.0.169

With the "dom-id" from the first command, execute the following command to get the VNC port:

[root@xengz ~]# xenstore-read /local/domain/3/console/vnc-port
5902

Now, if you are in the same network, just fire up your desired VNC client against 192.168.0.169:5902
However, if you are located in different network, you can create a ssh tunneling:
ssh -L 7777:localhost:5904 root@xen_ip

...and connect with your VNC client to localhost:7777

END!