Which one should I prefer?
Is it worth it?
Does it worth it?
Additionally, is the following form (without it) correct?
Is it worth?
"Is it worth it?" is correct.
No, that's wrong, worth always needs another word, like "worth twenty pounds" or something.
Any sentence structure that links "to be" and "worth" must have something to link it to. This is worth ten dollars. Is this worth more than that? My pen is worth only fifty cents. "Worth" in this case is an adjective meaning "valued at." My pen is valued at only fifty cents.
The question, is it worth it is really asking for the link, or in other words, the defined value. Is this worth ten dollars? Is this worth the effort it would take to do it? In the question, "Is it worth it?" the second "it" is standing in for the value. "Is it worth it (it being the amount of effort that would be required)?"
But "worth" is also a noun meaning "value," and in this case you may show this linkage, but you don't have to, and often we don't. Its worth is inestimable. It has tremendous worth. The worth of communication lies in the understanding of other human beings.