Memristors with excellent scalability have the potential to revolutionize not only the field of information storage but also neuromorphic computing. Conventional metal oxides have been widely used as resistive switching materials in memristors. Interface-type memristors based on ferroelectric materials are emerging as alternatives in the development of high-performance memory devices. A clear understanding of the switching mechanisms in this type of memristors, however, is still in its early stages. By comparing the bipolar switching in different systems, it is found that the switchable diode effect in ferroelectric memristors is controlled by polarization modulated Schottky barrier height and polarization coupled interfacial deep states trapping/de-trapping. Using semiconductor theories with considering polarization effects, a phenomenological theory is developed to explain the current-voltage behavior at the metal/ferroelectric interface. These findings reveal the critical role of the interaction among polarization charges, interfacial defects and Schottky interface in controlling ferroelectric resistive switching and offer guidance to design ferroelectric memristors with enhanced performance.