At unmagnetized planets like Venus, the only barrier for the solar wind is formed by the relatively weak induced magnetosphere, caused by the interplanetary magnetic field. Thus, the solar wind reaches regions near the planet and can directly interact with lower regions, sometimes even with the ionosphere.One possible interaction is the formation of the Kelvin-Helmholtz instability (KHI) at boundary layers around Venus. Due to the velocity shear between the upper and the lower region, small initial perturbations may grow and form vortices and possibly plasma clouds, leading to loss of ionospheric particles.Recent numerical studies investigated the influence of the density increase towards the boundary layer and of gravity on the growth of the KHI. In this work we focus on the effect of a magnetic field component parallel to the flow, which is known to stabilize the KHI from theory. A parallel magnetic field also leads to configurations that allow possible magnetic reconnection processes, which are also investigated in this thesis. Finally we compare our 2.5D magnetohydrodynamic simulation data with Venus Express measurements.Our simulations show that the stabilization due to a parallel magnetic field component is relatively high. For the growth of the KHI, the magnetic field has to be nearly perpendicular to the flow, otherwise the growth rates become too small for the evolution of significant Kelvin-Helmholtz vortices. A small magnetic field component parallel to the flow leads to reconnection processes and to the formation of islands of solar wind material inside the ionosphere. However, we do not observe the formation of clouds of ionospheric plasma in the solar wind region. The comparison of our results with measurement data shows that the observed structures in the magnetic field data indeed may be caused by the KHI around the planet.