In example http://openlayers.org/en/latest/examples/draw-and-modify-features.html when you hover on the center of the circle appears a point and you can move circle. Is it possible to do the same for the polygon?
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I'm not sure of your expectations from your description. As a circle has a center to draw it, it differs from getting a center in a polygon to drag it.
From my viewpoint, you should look at this other official demo to translate/move features like polygons.
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I think you need to check this example. --> ol3 example draw shapes
You can draw squares, rectangles, or even stars
