DV is large for its quality and does not open on phones or in browsers. MKV can carry several audio and subtitle tracks in one file. A DV file usually holds DV video (intraframe, about 25 Mbit/s) with PCM audio. DV is at home in capturing and archiving MiniDV tapes; MKV is the usual choice for films with several audio and subtitle tracks.
The defaults suit most files. Under Options you can lower the resolution, trade quality against file size, remove the sound and trim the start and end. This is demanding work for a browser: a long or high-resolution file can take several minutes, and your device does all of it, so keep the tab open.
This pair uses the compatibility engine, ffmpeg built for the browser, because DV is not handled by the browser's built-in codecs. It is a one-off download of about 10 MB that the browser keeps, it is slower than the built-in codecs, and it takes files up to 1 GB.