When you shut down a server in XenCenter, any virtual machines (VMs) running on it are shut down, and then the server is disconnected and powered off. If the server is a member of a pool, the loss of connectivity on shutdown will be handled and the pool will recover when the server returns. If you shut down another pool member (not the master), the other pool members and the master will continue to function. If you shut down the master, the pool will be out of action until the master is rebooted and back on line, at which point the other members will reconnect and synchronize with the master, or until you make one of the other members into the master (which you can do via the XenServer xe command line interface (CLI)).
Note that VMs with XenServer Tools installed ("paravirtualized" VMs) will be shut down gracefully, but VMs running in HVM mode (that is, VMs without XenServer Tools installed) will be shut down using a forced shutdown; to avoid this, you should install XenCenter tools on all HVM virtual machines.
After you power the server back on, you will need to connect to it again - see Reconnect a server.
To shut down a server
Select the server in the Resources pane and then click Shut Down on the toolbar.
When the server has been shut down, its status in the Resources pane changes to Disconnected .