Can new permanent residents of the United States enter Canada without a green card (with an American immigrant visa)?

Yes, but don't do it first. Second, you can't enter Canada at any time with an American immigrant visa.

About the first one, why don't you do it. After you enter the country, it only takes about 4 weeks to get a green card, and some of them are 1 week. If you leave the country during this period, it will be difficult for you to explain why you returned to the United States later.

On the second point, even if you get a green card, you are still a citizen of China. If there is an agreement between the United States and Canada, visa holders of that country are mutually recognized to apply for tourist visas of the other country. While you are in the United States, you can also apply for a Canadian tourist visa to go to Canada, but that is based on your China passport and has nothing to do with your American green card. In other words, the Canadian Embassy in the United States issued a visa to a China citizen in the United States, not a permanent resident of the United States.