Many times you can find difficult to perform simple tasks if you do not have the appropriate documentation, or you are not searching it correctly :)
It happened to me the other day: I had to rename a vm in Citrix Xen without XenCenter. After few readings, I discovered!
Read on.

[root@XenGz xen]# xe vm-list
uuid ( RO)           : 8ed02ab3-854d-5b75-ee10-194ed204a241
     name-label ( RW): gzvm64
    power-state ( RO): halted

[root@XenGz ~]# xe vm-param-set uuid=8ed02ab3-854d-5b75-ee10-194ed204a241 name-label=prodvm64
[root@XenGz ~]#         [color=yellow]<- no output![/color]

[root@XenGz xen]# xe vm-list
uuid ( RO)           : 8ed02ab3-854d-5b75-ee10-194ed204a241
     name-label ( RW): prodvm64
    power-state ( RO): halted