While plc, plf, pli, and plfp plot representations of a single valued function of two variables, the plv command plots a 2D vector at each of a number of (x,y) points. The vector actually looks more like a dart -- it is an isoceles triangle with a much narrower base than height, with its altitude equal to the vector (u,v), in both magnitude and direction, and its centroid at the point (x,y):
Making a good vector plot is very tricky. Not only must you find a nice looking length scale for your (u,v) vectors -- the longest should be something like the spacing between your (x,y) points -- but also you must sprinkle the (x,y) points themselves rather uniformly throughout the region of your plot. The time you spend overcoming these artistic difficulties usually isn't worth the effort.