Hey, great question... in my native speaker Australian context they are so close in definition that it is acceptable to use them interchangeably in your given example i.e. I�m certain/positive

Certain generally means that you are almost convinced that something is true

Positive, used in the same context, generally means that you are definitely convinced about it

I�m certain that he plagiarized his essay (meaning that it looks plagiarized but you don�t know from where)

I�m positive that he plagiarized his essay (meaning that you are definitely convinced and you have some indication where he plagiarized it from)

If your university dean then asks you how convinced you are, you can say (using the same meanings above)

I�m certain of this

I�m positive of this

Just to confuse you, you can also use them together although this is rare: I�m positively certain that he plagiarized his essay!