This paper proposes a controller which stabilizes a fixed-wing Unmanned Air Vehicle (UAV) in hovered flight. A six degree-of-freedom dynamic model is developed for an underactuated model-scale aircraft and a simpler three degree-of-freedom model is derived for controller design purposes. A novel nonlinear controller which renders the hovering position of the simplified system dynamics almost globally exponentially stable is proposed. Lateral stabilization is achieved by means of classic linear optimal control techniques. The nonlinear controller is designed so as to provide unidirectional thrust. Moreover, the closed-loop system is Input-to-State Stable (ISS) in the presence of unknown disturbances. Realistic simulation results demonstrate the controller's effectiveness and robustness when dealing with the problem at hand.