what is residual fm
Residual FM is a term that is used less today than it used to be. Residual FM and phase noise are related only that if you have low phase noise, you probably also have low residual FM.
But like said above, phase noise is a measurement made at many different offset frequencies. This is much more meaningful for modern communications systems. For example, if you have a low data rate system using phase modulation (qpsk for instance), you only care about the phase noise close to the carrier. If you take that same qpsk system and try to send 50 MBPS data on it, you really only care about phase noise far from the carrier (further than 10 KHz from the carrier).
Residual FM is basically an integrated snapshot of phase noise, so there is no information of the noise shape. It was a good measurement when systems used only FSK modulation.