How many gallons is your pond? What kind of filtration do you have, other than the UV? I'm not a big proponent of UV filtration. It does clear green water, but what else is it killing in the meantime? Running two UVs and still struggling with green water tells me either your UV isn't working - bad bulbs maybe? - or the water is flowing too fast past the UV.
Unfortunately emptying and refilling a pond is counterproductive to getting rid of any kind of algae - you're just forcing your pond to start all over to seek balance. Any kind of algae is a sign of too many nutrients in a pond - too many fish, too few plants, decaying organic material in the pond, overfeeding, etc. Figure out the problem and address it and you'll not be in a constant battle for clear water. Barley straw supposedly works by releasing h2o2 as it decays. But then you have decaying organic material in your pond - never made sense to me. Cut to the chase and add the hydrogen peroxide directly.
Remember that the green water doesn't bother your fish one bit. Water clarity is NOT a sign of water quality which is all your fish are concerned about. And, in fact, that green water may be what's keeping your fish healthy in your pond as it is consuming the excess nutrients that would otherwise be harmful to your fish.
Sunlight is always blamed for algae growth. In my experience sunlight makes algae GREENER, but it doesn't make it grow. My pond is in full sun all day and we've never had green water. Shading a pond isn't a bad idea - fish appreciate the shade and cooler water - but it won't stop algae from growing if you have a pond that's out of balance.
