Strong hysteresis in the I-V characteristics of organic thin film transistors are a severe obstacle for the implementation of large circuits. It therefore is a key success factor for the optimization and widespread application of organic electronics to understand the underlying principles. We report the fabrication of two types of pentacene transistors with either polyvinyl alcohol (PVA) or SiO2 as gate dielectric. These devices respond to transient measurement sweeps with a fundamentally different I-V hysteresis. A self-contained model is presented, which associates this behavior with the influence of traps at the SiO2/pentacene interface and polarization in the PVA layer. Simulations employing the commercial drift-diffusion tool SENTAURUSTM are performed to verify our models.