Being an Introvert is just a point on a scale of the way people socialise and get their social needs met - from the energy of others, collaboratively, or alone and more reflectively. It isn't a protected characteristic or a SEN.
I am bemused by the frequent view on MN that introverts are the hapless victims of evil extroverts. OK, extroverts can be a PITA but so can introverts - often not much would happen without the initiative or motivation of extroverts. There is a theatre exercise about making eye contact. Interaction builds between those who make eye contact with others who do and also those who don't. Nothing happens between two non-contacters. They are isolated - OK maybe that is a preference, but also not part of any building of a community or society. That is left to extroverts and those who interact with extroverts. I get a bit eye rolley about this special status afforded to those who can't or choose not to interact socially.
(I am probably a middle-way sort of person)
Paula presumably has no one else to be with at Christmas.
Same as Joan.
Both are friends. Both equal of a welcome in YOUR home as YOU choose.
However, to be pragmatic, I might have a drink with Joan first to let her unload what has been happening wrt her break up as that wouldn't be ideal as Christmas Day conversation centrepiece for any extended time.
But Paula presumably will not dissolve or burst into tears or be utterly miserable because you include Joan. She might even have sympathy for Joan and understand you wish (if you do wish) to include her.
Don't second guess Paula or be patronising about her ability to have a good time alongside Joan.
Look at your people pleasing tendencies in the middle of all this. (that is a note to myself - I would need reminding of that)
Good luck - I hope you have a lovely Christmas whatever you decide.