Storage on XenServer VMs is provided by virtual disks. A virtual disk is a persistent, on-disk object that exists independently of the VM to which it is attached. Virtual disks are stored on XenServer Storage Repositories (SRs), and can be attached, detached and re-attached to the same or different VMs when needed. New virtual disks can be created at VM creation time (from within the New VM wizard) and they can also be added after the VM has been created from the VM's Storage tab.
Virtual disks on paravirtualized VMs (that is, VMs with XenServer Tools installed) can be "hotplugged", that is, you can add, delete, attach and detach virtual disks without having to shut down the VM first. VMs running in HVM mode (without XenServer Tools installed) must be shut down before you carry out any of these operations; to avoid this, you should install XenServer Tools on all HVM virtual machines.
On the VM's Storage tab in XenCenter, you can: