The conserved quantity is mass-energy.

1. A particle at rest has only (rest) mass.

2. A particle near the speed of light, such as a typical neutrino, has some rest mass (with Erest given by mrest*c^2) and kinetic energy K >> Erest.

3. A massless particle, such as a photon has no rest mass, no Erest, only its "kinetic" energy pc (=hc/lambda).

In a collision where some particles are destroyed and others created, the total mass-energy is conserved, but it can be distributed in various combinations of rest masses and kinetic energies.

The conserved quantity is mass-energy.

1. A particle at rest has only (rest) mass.

2. A particle near the speed of light, such as a typical neutrino, has some rest mass (with $E_{rest}$ given by $m_{rest}c^2$) and kinetic energy $K \gg E_{rest}$.

3. A massless particle, such as a photon, has no rest mass, no $E_{rest}$, only its "kinetic" energy $pc = \frac{hc}{\lambda}$.

In a collision where some particles are destroyed and others created, the total mass-energy is conserved, but it can be distributed in various combinations of rest masses and kinetic energies.