Networks, PIFs, and VMs can be hidden from XenCenter by adding the key HideFromXenCenter=true to the other_config parameter for the object in the XenServer API. For example, certain VMs could be hidden because they shouldn't be used directly by general users in your environment. Objects flagged with this key are hidden objects, and by default they will not appear anywhere in XenCenter.
To make hidden objects visible in XenCenter, on the View menu, click to select Hidden Objects. To hide them again, on the View menu, clear the Hidden Objects check mark.
See the XenServer Software Development Kit Guide to find out more about flagging objects using the HideFromXenCenter key.