Passive synthetic aperture radar imaging uses illuminators of opportunity and moving receivers to image the scene of interest and has good electronic antagonism. It can also reduce system cost, benefit to system miniaturization and ease frequency band congestion. In this paper, we present a unified theory for passive synthetic aperture radar imaging based on inverse scattering and microlocal analysis. It is suitable for airborne receivers flying along arbitrary flight trajectories and static or moving illuminators of opportunity. Two types of waveforms are considered: narrowband continuous-wave (CW) waveforms and wideband pulsed waveforms. Our theory results in two novel synthetic aperture imaging modalities: Synthetic aperture hitchhiker (SAH) that uses wideband pulsed waveforms and Doppler synthetic aperture Hitchhiker (DSAH) that uses single-frequency or ultra-narrowband CW waveforms. First, we develop measurement models which are able to remove the transmitter-related terms in the phase of the correlated measurements. We use filtered backprojection (FBP) theory and microlocal analysis to develop approximate inversion formulas for SAH and DSAH. The inversion formulas involve backprojection of the correlated measurements onto iso-passive-range and iso-passive-Doppler contours for SAH and DSAH imaging, respectively. Detailed resolution analysis is given. Finally, we present numerical simulations to demonstrate our theoretical results.