Here is an design recommendation from Atmel about implementing USB on AVR XMEGA microcontrollers. The article provides all the basic information you’ll need to get started with designing USB devices with the XMEGA.
This document is written for hardware designers to help them develop their applications. It assumes that readers are familiar with the AVR XMEGA architecture. A basic knowledge of the USB 2.0 specification (www.usb.org) is also required to understand the content of this document and compliance constraints.