The kit is powered via the USB connector, which leaves two options to power it: Connect the kit either to a PC through a USB cable or to a 5V USB power supply (AC/DC adapter).
The 5V is regulated down to 3.3V with an onboard LDO regulator, which provides power to the entire board. The ATxmega128B1 is powered by 3.3V, but if operation at lower voltage (1.8V min) is desired, some modifications to the board are needed. This includes replacing the regulator with one that delivers the expected voltage and rerouting the power to the device (see schematic for explanation). As some of the other ICs on the Atmel XMEGA-B1 Xplained require 3.3V to operate correctly, these devices have to be removed also.