The USB port on the Motorola XOOM device is for data transfer only and does not have any charging capability.
The first step is to ensure you are using the proper charger to charge your device. The Motorola XOOM tablet will only charge with the travel charger or vehicle charger designed for the Motorola XOOM tablet. You can also charge through a dock station connected with the power charger plugged into an electrical outlet.
No it will not charge over USB, the USB port is for data transfer only. You can find the Xoom owners manual on line thru google and it will confirm what I said.