We have developed a three-dimensional, self-consistent full-quantum transport simulator for nanowire field effect transistors based on the eight-band k center dot p method. We have constructed the mode-space Hamiltonian via a unitary transformation from the Hamiltonian discretized in the k-space, and reduced its size significantly by selecting only the modes that contribute to the transport. We have also devised an approximate but highly accurate method to solve the cross-sectional eigenvalue problems, thereby overcoming the numerical bottleneck of the mode-space approach. We have therefore been able to develop a highly efficient device simulator. We demonstrate the capability of our simulator by calculating the hole transport in a p-type Si nanowire field effect transistor and the band-to-band tunneling current in a InAs nanowire tunnel field effect transistor.