Why ethtool cannot get information or do operation in xen kernel

Issue

I am in xen kernel. I execute command like

ethtool -s speed 100

but return the result

[root@dhcp-0-104 ~]# ethtool -s eth0 speed 100
Cannot get current device settings: Operation not supported
  not setting speed
[root@dhcp-0-104 ~]#

and I use

ethtool eth0

to get the ethernet card's information, but just return me few information.