Student Visa is only issued to students who come to Russia for an exchange or to study. The "foreign department" of the university, school or institution can issue the invitation and should take care of all the formalities. They will submit your data to the immigration officials. If the Russian Consulate in your country does not require you to present the original invitation, a copy can be given to the Consulate for getting the applicant a tourist visa.
A student visa doesn't usually have a limited validity, and can be extended for the period of study.
