Electric violin is simply a violin with an electronic signal output.
The term can refer to an acoustic violin with an electric pickup
of some type, but usually refers to a solid-body electric instrument.
The solid body violins typically have a non-traditional, minimalistic
design to keep weight down.
Acoustic violins may be used with an add-on piezoelectric bridge
or body pickup, but often suffer from feedback on stage, in addition
to the raw piezo sound. Some magnetic pickups have less sharp
sound and no feedback. To prevent this feedback from the resonances
of the hollow body under high amplification on stage, many instruments
have a solid body, which helps reduce or eliminate such feedback.
The timbre or tone color of an acoustic violin is created directly
because of these resonances, however, so depending on how the
signal is picked up, an electric piezo violin may have a much
more "raw", "sharp" sound than an acoustic.
Electric violin signals usually pass through electronic processing,
in the same way as an electric guitar, to achieve a desired sound.
This could include delay, reverb, chorus, distortion, or other
effects.
Since it (usually) has metal strings, the sound of the violin
can be sensed with either magnetic or piezoelectric pickups. Magnetic
pickups generally may have a less sharp sound and less feedback.
Magnetic setups similar to those used on electric guitars are
few, but one unusual violin system is using the strings as a linear
active pickup element.
Generally, piezo pickups are more common because of very cheap
piezo materials available. They are used to detect physical vibrations,
sometimes in or on the body, but more commonly in the bridge.
Some piezo setups have a separate pickup (or two!) embedded in
the bridge under each string.
Piezo pickups have a very high (capacitive) output impedance,
and require a powered preamp for filtering the raw sound (a charge
amplifier is best, and tube-driven is sometimes favored over solid
state for the purpose of desired distortion), and to avoid signal
loss and excessive noise pickup in the instrument cable. A solid
body provides room for this circuitry and a battery, although
phantom power can make the battery unnecessary.