Why do some countries still use heavy water reactors when there are light water ones? Are heavy ones the only ones which produce nuclear weapon material? If so, isn't this a hint to those people who believe that Iran is peaceful?
Heavy-water reactors can run with unenriched uranium (which Iran has in spades). Light-water reactors require moderately-enriched uranium (of course, it's not all that hard to go from moderately-enriched to highly-enriched when the IAEA wants you to have nukes under the table).
Are heavy ones the only ones which produce nuclear weapon material?
No, but because there is more U-238 in unenriched uranium, heavy-water reactors produce more plutonium (specifically Pu-239, the bomb-making isotope) than light-water reactors.
Three possible reasons:
1) A heavy water reactor running on U-235 can "go critical" with natural (aka "un-enriched" uranium), and produce power and plutonium.
2) A heavy water reactor fueled with a mix of enriched U-235 and thorium will "breed" U-233 from the thorium (the U-233 is also fissionable, and can then be used to fuel for "burner" reactors). This technology is being developed by India.
3) A heavy water reactor fueled with enriched U-235 plus U-238 will breed plutonium in larger amounts.
All three have peaceful uses---but #3 points HEAVILY to the production of plutonium for use in fission explosives.
No, heavy water reactors allow the use of unenriched uranium in the fuel cycle, and easy recycling of reprocessed spent fuel. Do a web search for the 'CANDU' reactor design.