PABX Private Automatic Branch Exchange.
PBX: Private Branch Exhange
I read some years ago bout differences, I found that PABX is like a small exchange with functions like a Public Exchange, it is oriented to private networks, PBX is a small device that allows to do calls with poor set of features for the lines. At the end of the day I have seen that PABX and PBX becoming the same, because more features are included in the PBX. Hope this could help, more thoughs or corrections are welcome.