Difference Between Linear Convolution and Circular Convolution
The difference applies only to the borders of the image.

In the linear convolution you assume the values of pixels beyond the border (examples being mirror of the image pixels, or 50% grey).

In the circular convolution (or DFT, product, IDFT), the pixels beyond the border are the pixels on the other side of the image, just as if you had a repeated tiling of the image.