Below are steps
1. Using SSH Login to your server. You can use putty software to login to server from Windows Machine.
2. For example : Consider you wish to add /dev/sdb drive which is newly attached to system. If this is so then give following command on # Prompt.
# xe sr-create name-label="HardDisk-B" shared=false device-config:device=/dev/sdb type=lvm content-type=user
3. Just have a look at Xen Center. You can see this new hard drive there.