Using 'kind of' says that you may be kidding about some aspects of what you just said, but you may not be kidding about all of it. Saying 'I'm just kidding' means exactly that. We also use 'kind of' in other situations such as when you are asked if you like something or someone. You can reply 'yes, kind of' which leaves you with options to say a definite 'yes' or a 'no' later.

the addition of "kind of" means that the speaker is not entirely kidding. If somebody says "Your cooking is bad! Just kidding... kind of.." It means while they actually do think your cooking is a little bad. Hope this helps :)

Not there is a noticeable pause between "I'm just kidding" and "kind off" To initiate a rollercoaster of emotion. For example: A: "Your cooking is horrible!" B: (emotion: shocked or jarred) A: "... just kidding." B: (emotion: relieved) A: "... kind of" (meaning, I really do mean what I first said, at lease in part.) B: (emotion: disappointed) I'm reminded of the chat between Jodi and Alison Scott in the kitchen scene from the movie "Knocked Up".