Disclaimer: I am not a dev, my technical understanding is limited, and I only discovered kbin today.

The difference is that they are running completely different software, despite speaking the same* language ('protocol'). There may be some things one software does that another can't. I wonder if it's easier to answer what distinguishes it, or what makes them similar. They're both link aggregators (the same kind of website as reddit, e.g. people post links to groups and they get voted up or down by subscribers), and they're both able to process each other's posts and see each other's groups (kbin calls them magazines, apparently, while we call them communities. I don't know if that's purely semantic or if there is a profound difference). So as far as basic usage goes, both can make a post and unless they do something fancy, the other site can read and reply to it.

kbin has a different visual layout and appears to have more focus on also having microblogging and social media features within those groups, we don't have that feature and integration with Mastodon can be a bit stranger here (such as them replying to replies, in my experience it doesn't nest neatly like ours do, instead just showing as a reply to the original post, and maybe that's unavoidable when a twitter-like thread without proper comment replying has to fit our comments layout). It seems Lemmy has stayed closer to what reddit is like, while kbin has strayed into a more experimental approach.

kbin says "This is a very early beta version, and a lot of features are currently broken or in active development, such as federation" (and I have noticed the federation doesn't show some posts yet which would be expected to show). Lemmy doesn't seem to have such disclaimers. It's current version number suggests it is considered more mature, but still not particularly stable either.

kbin seems to have a mobile app under development, while Lemmy's seem to be more mature. That said, I've never used one.