This, in fact, is a very complex question. In India, there are many different types of visa, for example for tourists, businessmen, journalists and researchers. International interns come to India with several different types of visa: Tourist Visa, Entry Visa, Employment Visa, Business Visa or Student Visa, none of them applicable to all internship scenarios.
The entry visa (X Visa) is generally for people whose purpose of visiting India does not fit into any of the standard visa categories, such as people coming to short-term courses, volunteer work or unpaid internships to India. You must apply for an entry visa along with a copy of the offer letter stating the terms of your internship position. If you are a student and the internship is part of your studies, you should also get a letter from your university explaining why you are going to India.
Before applying for any type of visa, make sure that your passport is valid for at least 6-18 months, depending on your specific visa. Applicants who are not citizens of their country but residents must send a copy of their residence permit with the application. It is always advisable to consult the respective embassy in your country, for example, the India visa application embassy in Spain, Germany, Italy, etc. There you will find most of the information considering your visa.