Is it discrimination? No.
They didn't schedule the party so as to specifically exclude you. They scheduled it to accommodate the majority of the employees and to minimize any impact to the business if the party is scheduled during work hours.
Don't take it personally. If you can attend, then do so. If you can't, then don't.
In short, yes and yes (though not directly).
First of all, allow me to say that although it does appear as if your boss is not the perfect boss, you are truly being the issue. Forgive me for saying it so directly. Consider your attitude, and pay attention to the wording when reflecting on what you wrote yourself. I've added some emphasis:
Me [and my boss]...
So If I understand correctly, the Christmas party starts during the afternoon and will probably extend to after work-hours? You only work the morning? Attendance is optional?
So nobody will get any more pay then what they would usually get.
Nobody gets any less pay then they would usually get.
Everybody gets free drinks and food.
Some people get some hours ...
My situation is a little different to others
I really don't think so. This situation is literally the thing everybody on YouTube is making fun of when they make fun of Millenials in the workplace.
Me and my boss do not have a very good relationship on a personal level, we often argue about the morals and future of the company, he is the sort of person ...