"No kidding" and "not kidding"

What is the difference between not kidding and no kidding?

Is no kidding valid English?

The phrases mean roughly the same thing in most contexts, but are used slightly differently (not kidding requires mentioning who isn't kidding, whereas with no kidding it is implicit). There are many ways these phrases can be used in colloquial English, including:

This may be hard to believe, but I'm honestly not joking:

There was a goose in our kitchen - no kidding! (or I'm not kidding!)

Really? You're not joking?

-There was a goose in our kitchen!

-No kidding?! (or You're kidding me?!)

I couldn't agree more; Yes, I know, I experienced it myself.

-It must have been hard for you to go without food for six weeks!

-No kidding! (or You're not kidding!)

Don't fool around

No kidding around now please!

"No kidding" means "I agree," while "not kidding" means "I'm not joking." "No kidding" could also mean "no joking allowed."

It is valid English, but only in certain registers.