CVU file is a CVSD Unfiltered Audio. Continuously Variable Slope Delta modulation (unfiltered) is an alternative handler for CVSD that is unfiltered but can be used with any bit-rate.
MP3 is an audio format that can compress and encode an audio file. It uses the lossy compression algorithm to contain audio data. The lossy compression helps to reduce the data significantly without losing audio quality. MP3s are created using different bit rates maintaining the original quality.