We investigate and experimentally demonstrate an optical signal-to-noise ratio (OSNR) monitoring technique for polarization-division-multiplexed return-to-zero ON-OFF keying (PDM-RZ-OOK) and differential-phase-shift-keying (PDM-RZ-DPSK) signals. Both polarization tributaries of the PDM channel can be simultaneously monitored based on cascaded four-wave mixing in a polarization-diversity nonlinear loop mirror. The interchannel crosstalk induced by nonlinear polarization scattering could be mitigated using the polarization diversity loop structure. As OSNR varies between 5 and 20 dB, the output power of the monitor has a dynamic range of 4.2 and 4.6 dB for the OOK and DPSK signals, respectively. We also demonstrate that the performances of the monitor are the same for both time-interleaved and synchronized PDM-RZ signals.