Quoted from above link:
"The USB 3.0 stack is new in Windows 8. Microsoft created the new drivers by using Kernel Mode Driver Framework (KMDF) interfaces. The KMDF driver model reduces complexity and improves stability.
USB 3.0 host controller driver (Usbxhci.sys)
The xHCI driver is the USB 3.0 host controller driver. The responsibilities of the xHCI driver include initializing MMIO registers and host memory-based data structures for xHCI controller hardware, mapping transfer requests from upper layer drivers to Transfer Request Blocks, and submitting the requests to the hardware. After completing a transfer, the driver handles transfer completion events from the hardware and propagates the events up the driver stack. It also controls the xHCI controller device slots and endpoint contexts.
The xHCI driver is new in Windows 8 and is not an extension of the eHCI miniport driver that was available in earlier versions of the operating system. The new driver was written by using Kernel Mode Driver Framework (KMDF) interfaces and uses KMDF for all controller power management and PnP events. Windows loads the xHCI driver as the function device object (FDO) in the device stack for the host controller.
USB host controller extension (Ucx01000.sys)
The USB host controller extension driver (an extension to KMDF) is the new extension to the underlying class-specific host controller driver, such as the xHCI driver. The new driver is extensible and is designed to support other types of host controller drivers that are expected to be developed in the future. The USB host controller extension serves as a common abstracted interface to the hub driver, provides a generic mechanism for queuing requests to the host controller driver, and overrides certain selected functions. All I/O requests initiated by upper drivers reach the host controller extension driver before the xHCI driver. Upon receiving an I/O request, the host controller extension validates the request and then forwards the request to the proper KMDF queue associated with the target endpoint. The xHCI driver, when ready for processing, retrieves the request from the queue. The responsibilities of the USB host controller extension driver are:
Provides USB-specific objects to the xHCI driver.
Provides KMDF event callback routines to the xHCI driver.
Manages and control the operations of the root hub associated with the host controller.
Implements features that are configurable by the client driver, like chained MDLs, streams, and so on.
USB hub driver (Usbhub3.sys)
The new hub driver, in the USB driver stack for 3.0 devices, uses the KMDF driver model. The hub driver primarily performs these tasks:
Manages USB hubs and their ports.
Enumerates devices and other hubs attached to their downstream ports.
Creates physical device objects (PDOs) for the enumerated devices and hubs.
Windows loads the hub driver as the FDO in the hub device stack. Device enumeration and hub management in the new driver are implemented through a set of state machines. The hub driver relies on KMDF for power management and PnP functions. In addition to hub management, the hub driver also performs preliminary checks and processing of certain requests sent by the USB client driver layer. For instance, the hub driver parses a select-configuration request to determine which endpoints will be configured by the request. After parsing the information, the hub driver submits the request to the USB host controller extension or further processing. "
