I'm not interested in you.
I'm interested in you.
Is this in passive form? If not, please explain to me what structure it is following.
Yes, you can view it as a passive form of the sentence You interest me. However, the proper passive form of this sentence is I'm interested by you.
On the other hand, you can say that interested is an adjective and that interested in sb. is an adjective (deverbal) phrase.
So yes, you can view it as a sentence in passive voice, but if you wish, you can view it as a descriptive sentence that says: I'm a person with a specific property: interest in you. Since the proposition has changed and since the "passive" form is more common than the "active" form of this statement, I would be inclined to the latter explanation.
The construction of passive voice: be+past participle. However, your sentence is something different. It says: be+adjective.
Take a look at the following examples:
I am happy (=adjective) with you. (Active sentence) I am interested (=adjective) in you. (Active sentence)
However, "I am interested (=past participle) by her" is an incorrect sentence.