Just from what I understand myself, it's that they are two different software setups. But they both use the ActivityPub standard that all federated content is using. KBin is different though in that it's trying to be more like Twitter with Sub Reddits, than like Reddit with Tweeting. And Lemmy is just purely trying to recreate the Reddit experience. So like on Twitter, Mastadon, and KBin upvote is more of a like and people can see what accounts "like." KBin and Mastadon share the same 'microblogging' feature which is like twitter. From what I understand they share microblogs across the fediverse.
And Lemmy and Kbin share communities/magazines together thanks to federation. So you're on a magazine in KBin but I'm on Lemmy on Lemmy World looking at this community from Lemmy.ml interacting with you.
Overall I think if you like Twitter and Reddit and are fine with your entire history of actions being public KBin is perfect. If you just want a Twitter experience, Mastadon. If you want some more obscurity with your account like Reddit and only the Reddit experience, Lemmy.
Right now I think KBin's feature parity isn't too far off from Mastadon and Lemmy. But like the rule of any thing that combines two tools, it can't be better than both separately. I think as Lemmy and Mastadon matures into their niches KBin will almost exclusively be playing catch up with both in the long term.