I found that arch provides both
oxygen-icons-svg, what's the necessity of svg format? Is there something that a PNG image "can't do", maybe performance issue?
Not sure about it.
svg is a vector based image format, so it'll scale to any size without pixelating. The downside that it takes more CPU power to render the image, so png is slightly there. Though for icon-sized images that shouldn't be very noticeable.
The "biggest" problem with svg is that not all applications support it, while they do support bitmap formats such as png.