When you create a virtual machine, it becomes part of the vCenter Server inventory and is registered to the host and vCenter Server. If you remove a virtual machine from the vCenter Server inventory, it becomes unusable. Virtual machine files remain in the same datastore but you cannot power on the virtual machine when it is not registered in the inventory.
You can temporarily remove a virtual machine from vCenter Server by unregistering it. Virtual machine files are not deleted from the datastore. Though all high-level information about the virtual machine such as statistics, resource pool association, permissions, and alarms, is removed from the host and the vCenter Server instance. To remove a virtual machine from the inventory, call the unregister(vm_ID) method of the VM service and pass as argument the ID of the virtual machine.
To restore a virtual machine to the vCenter Server inventory, and make it usable again, call the register(register_spec) method of the VM service. You pass as argument a VMTypes.RegisterSpec instance that contains information about the current location of the virtual machine files on the datastore. You can also define the location within the vCenter Server inventory, for example, the cluster, folder, or the host, where you want to register the virtual machine. After registration, the virtual machine takes its resources (CPU, memory, and so on) from the resource pool or host to which it is registered.
If you no longer need a virtual machine and you want to free up datastore space, you can permanently delete a virtual machine from the inventory. Call the delete(vm_ID) method of the VM service and pass as argument the ID of the virtual machine. Upon a successful completion of the operation, the virtual machine files are removed from the datastore, including the configuration file and the virtual disk files.
