Octave violins are exactly what they sound like, violins that are strung with octave strings so that they sound an octave lower. While you can put octave strings on a standard violin, in most cases it requires some modification to the nut, bridge and pegs. You can also use an octave pedal or any pitch shifter or multi-FX pedal that includes a sub-octabve effect, however, in addition to the fundemental pitch being transposed down, all of the overtones get transposed down an octave as well, making the effect noticable. For these reasons, the best and purist way to play an octave lower is to use a purpose-built octave violin.
For a true 'chin cello,' a violin that plays cello range, select a 5-string octave violin or restring a 4-string octave violin ADGC.