Of all the BSDs, I'm only a little familiar with OpenBSD through its OpenSSH fame, so I'm a little surprised to hear that a (probably) somewhat related project would use XML in a system call. In other words, if an OpenSSH release introduced XML as a wire format I'd assume it to be an April fools's joke. [1] But I guess OpenBSD and NetBSD are less related than I thought.
Free, Net, and Open BSD are all relatively early forks of the same 386BSD project from the early 90s. Each project has different goals and code has diverged between them, though they frequently pull in changes from on another's codebases.
FreeBSD is the most general purpose of the 3 and the most popular. It is also the only BSD out of the big 3 that doesn't utilize a global kernel lock, allowing for modern symmetric multiprocessing similar to Linux.
NetBSD is aimed at being extremely portable. It's sort of like the "Can it run doom" of the OS world. Just take a look at their list of ports: https://wiki.netbsd.org/ports/
OpenBSD is aimed at being secure. Exactly how realized this goal is is somewhat controversial. But regardless of that, security is the stated highest priority of the development team.
There's also DragonflyBSD, which was forked by Matt Dillon from FreeBSD following some personal and technical disagreements. It's since diverged pretty heavily from the rest of the BSD family. Given its very low market share in this category of already niche operating systems, it seems more like a pet project of Matt Dillon's, though I'm sure it has serious users.
