We study the electronic structure and electron tunneling in Fe/KNbO3/Fe multiferroic tunnel junctions (MFTJs) with asymmetric interfaces using density functional theory calculations. There are large induced magnetic moments on interfacial Nb and O atoms, which are related to the direction of the polarization in KNbO3 barriers. The complex band structure of bulk KNbO3 indicates that the evanescent states with Δ1 and Δ5 symmetry have the smallest decay rates within the gap. We predict the conductance for the antiparallel and parallel magnetization configuration. The lowest decay rate of the evanescent states in KNbO3 characterizes the conductance for the majority spin channel. The polarization orientation has a large influence on the conductance. The tunneling magnetoresistance in the Fe/KNbO3/Fe MFTJs is small for the two opposite polarization states. However we obtain a high tunneling electroresistance for the two magnetization configurations of the electrodes.