1 [eV] ≡ 1.6021892 · 10^-19 [Joule] ≡ 1.6021892 · 10^-12 [erg] .

The confusion is because the same units are used to denote two separate quantities which happen to have similar magnitudes for a commonly encountered spectral model, Bremsstrahlung emission.

1. the frequency ν, or wavelength λ, of a photon: As Planck discovered, the energy of a photon is directly related to the frequency ν,
   

   E = h · ν ≡ h · c / λ ,

   where h=6.6261760 · 10^-27 [erg s] is Planck’s constant and c=2.9979246 · 10¹⁰ [cm s^-1] is the speed of light in vaccum. When λ is given in [Ångström] ≡ 10^-8 [cm], we can convert it as

   [keV] = 12.398521 / [Å] ,

   which is an extraordinarily useful thing to know in high-energy astrophysics.

2. the temperature T of a gas or plasma: Here we look to thermodynamics, which relates the kinetic energy of random motion of particles in a gas to a gross property, the temperature of the gas,
   

   E = k_B · T ,

   where k_B = 1.3806620 · 10^-16 [erg K^-1] is Boltzmann’s constant. Then, a temperature in degrees Kelvin can be written in units of keV by converting it with the formula

   [keV] = 8.6173468 · 10^-8 · [K] ≡ 0.086173468 · [MK] .

It is tempting to put the two together and interpret a temperature as a photon energy. This is possible for the aforementioned Bremsstrahlung radiation, where plasma at a temperature T produces a spectrum of photons distributed as e^{-h ν / k_B T} and it is possible to tie the temperature to the photon energy at the point where the numerator and denominator have the same numerical value. For example, a 1 keV (temperature) Bremsstrahlung spectrum extends out to 1 keV (photon energy). X-ray Astronomers use this as shorthand all the time, and it confuses the hell out of everybody else.