What is the difference between APV I and APV II?
The difference between APV-I and APV-II "... lie in the signal-in space performance requirements..." (see this presentation, in particular beginning on page 12: ENAV presentation).

This means that APV-I/II refer to navigation signal performance requirements for APV approaches. Note #8 on page 13 of the presentation referenced above is clarifying:


  The terms APV-I and APV-II refer to two levels of GNSS approach and landing operations with vertical guidance (APV) and these terms are not necessarily intended to be used operationally.


Therefore, look at APV-II versus APV-I as providing for lower APV minimums based on signal performance (horizontal and vertical alert limits) at locations where this is possible. Similar in concept, but not in value, as the difference between RNP .3nm and RNP .1nm approaches.