Where do they keep record of graduated students? I'm wondering if it is possible to find out if someone is graduated from a University. Like, I could write on my CV that I graduated from Harvard, but then how can you prove it? Suppose I'm an excellent hacker, I could still fool people by doing clever tricks that make a counterfeit of a diploma or something. So I'm wondering if there is an official protected and verified database that records the information that a person is a graduate from a specific university?

Suppose that I'm an employer and I want to verify my candidate is a graduate of X university, how do I do that? What if the university is in Canada?
Information on awarded degrees is considered publicly releasable directory information in the US. Dates of attendance, for those who did not complete a degree, as well as honors (which imply a certain class rank or GPA) can be released as well.

Most schools sell this information to an aggregator, the National Student Clearinghouse. For a fee you can verify a degree by providing the student's name, date of birth, and school.

Harvard, specifically their school of Arts and Sciences, directs third parties to this database. 

Laws in Canada differ and require signed permission from the student. A similar aggregator, AuraData exists. 

Otherwise, the student can have the school's registrar send a document, on security paper and sealed, listing awarded degrees to interested parties. This is called an official verification and is essentially a transcript that does not contain course or grade information.

Finally, if the person has a graduate degree that involved a thesis, one can  check the school's online library catalog to see if one is listed.