So I installed the M&M 1 through 6 pack today, and I'm a little confused about something.

Never played these games before, so what's the difference between the World of Xeen and Swords of Xeen? I believe Swords is some kind of mod, but is it an entirely different game or just added content or what?

Also a separate desktop icon popped up for Worlds of Xeen and Worlds of Xeen (Full Speech). Does that mean it's just a difference between whether there's voice acting or not?

Swords of Xeen was a fan-made mod that the developers chose to include as V came out. It is a game in itself. Not really part of the continuity.

Can't tell you about the full speech option. I'd have to guess maybe speech causes compatibility problems for some and having a speechless version avoids that issue.

Originally GOG only supplied non speech version. However eventually they added speech version as well, so now you can choose which one you want to play. There are no compatibility problems since it's all dosbox 'n stuff.