LED forward voltages are larger than that for those operating in visible light.
Silicon diodes have about 0.6V forward voltage, red LEDs about 1.6V, blue about
3.3V. All diodes emit light when forward biased, its just that some are optimized
for it. Silicon diodes emit infra-red and are also sensitive to light, which is why
they are packaged in black epoxy normally.
Conventional current is a notional flow of positive charge around a circuit - the
fact that in metals the current is actually carried by negative electrons moving
the opposite direction can safety be ignored in nearly all circumstances as we
only care about the maths being right. In semiconductors and liquids charges
of either polarity may be involved in carrying current.