[想 + v] means [want to + v]
[要 + v] can mean either [want to + v], [demand to] or [need to + v]
[想要 + v] means [want to + v] or [attempt to + v]
When you want to say "need to" you can only choose "要" among the three
When you want to say "want to", you can use "要", "想要" or "想"; Using "想" makes it unambiguous because it doesn't contain other meanings
When you want to say "attempt to" you can only pick "想要" among the three
Note: With a clear context, you can deduct 要 means "want to" or "need to" in most cases
Examples:
我要去美國 = I want to/need to go to America
我要去美國度假 = I want/demand to go to America for a vacation (agoing on a vacation is something you want to do in most cases)
我要去美國公幹 = I need to go to America for a business trip (going on a business trip is something you need to do in most cases)