What is an immigrant visa?

No, a visa is a visa, and a green card is just an identification document, which means that you have permanent residency in that country. As long as you pass an immigrant visa, you can officially enter the United States. I'll give you a temporary green card first, and then you can apply for a permanent green card. What has nothing to do with the visa is that once the immigrant visa is signed, you can get the right to legally stay in the United States, that is, the green card certificate.