Which Kennedy half dollars contain silver?

Unlike the common mis-usage of "silver dollar" to refer to any $1 coin, the term silver half dollar is used only for a coin that actually contains silver.

Up to 1964 all US halves were made of an alloy of 90% silver and 10% copper. That was reduced to 40% silver from 1965 to 1970, and all halves from 1971 to the present are made of copper-nickel like dimes and quarters.