What the different between PVC and SVC?

Virtual circuits are usually permanent virtual circuits (PVCs), which means that they are defined for frequent and consistent use by the network. They do not change unless the network manager changes the network. Some common carriers also permit the use of switched virtual circuits (SVCs) although this is not common. Changing PVCs is done using software, but common carriers usually charge each time a PVC is established or removed. The difference between a PVC-Permanent Virtual Circuit and SVC-Switched Virtual Circuit is that an SVC must be reestablished each time data is to be sent. After that the SVC disappears.