I understand that the term MV is simply a follow on from the early days of x-ray generation where we understood the characteristics of the generated x-rays by the discharge voltage between anode and cathode (eg kV in a superficial/orthovoltage unit). The unit MV (and for that matter kV) is supposedly strictly applied to x-ray generation although I understand you can see it being used in describing a radionuclides when their energy exceeds 1MeV. (Khan 1994) The MV designates a heterogeneous photon beam spectrum produced by an electron beam (of the same number MeV) striking a target. For electrons we still use MeV as this represents their kinetic energy, we spread the ~3mm electron beam either by scanning or scattering foils which are thin enough that the vast majority are scattered rather than being slowed or stopped. Occasionally in earlier texts and papers you may see the use of MeV describing a photon beam, however I don't think this occurs often these days.

I think the key to this question is the fact that energy of the electrons emitted from a linear accelerator is mono-energetic. All electrons have the same energy in MeV. On the other hand, photons emitted from a breamsstrahlung device such as an X-ray tube or a linear accelerator has an energy spectrum with energies from 0 up to the energy of the electrons creating the bremsstrahlung. Photons emitted from a radioactive decay would properly be labeled MeV because they are mono-energetic.

It´s very simple. If you use a X-ray tube to produce X-rays you chose a voltage with the unit kV. These kV accelerate electrons in the tube to get the energy keV. They hit the anode and produce X-rays within a triangle spectrum with a maximum energy of keV.

In a linac you use electrons with MeV, hit the tungsten target and they produce a bremspectrum with a maximal energy of MeV.

So the result is: you characterize X-raxs from the tube by the used voltage and use the same diction for linacs. If you say keV or MeV you point at the energy of electrons or protons.

Example: 6 MV (synonym for X6) means photons

6 MeV electrons synonym with E6 means electrons.

The energy of diagnostic and therapeutic X-rays is expressed in kV or MV. Whereas, the energy of therapeutic electrons is expressed in terms of MeV.

In first case, this voltage is the maximum electric potential used by a linac to produce the photon beam. The beam has a spectrum of energies: the maximum energy is approximately equal to the beam's maximum electric potential times the charge of the electron.

Thus, 1 MV beam will produce photons of no more than about 1 MV. The mean X-ray energy is only about 1/3 of the maximum energy. Beam quality and hardness may be improved by special filters, which improve the homogeneity of the X-ray spectrum.

1 MV [megavolt] = 1000000 volts

– is a unit of electrical potential.

1 MV [mega-electron-volt] = 1000000 electron-volts

= 1.60217653 x 10-13 joules = 0.16 picojoules

– is a unit of energy

A 6 MV X band linear particle accelerator (LINAC) must be able of delivering an electron beam of 0.5 MW peak at 6 MeV, in order to produce an X ray dose of 10 Gy/min at 80 cm for medical applications.

[1 Gy = 1 J/kg; 1 gray = 1 joule/kilogram]

X rays produced by LINAC have heterogeneous energy spectrum because of bremmstrahlung effects. thus, we can't use units of MeV, but we can say potential which electrons are accelerated.  

Aditional Tip: if your beremstrahlungs spectrum has a maximal6 enegy of A MV he medium photon energy has the value Emax/2. Example: X6 relates to a medium photon energy of 3 MeV. Not vryexact, but helpful.

A 6 MV photon beam would constitute beams with energies ranging from 0 to 6MeV. The 'average energy' of such a beam is about 1.5 MeV when using a LINAC. (this heterogeneous nature is due to it's mechanism of creation: bremsstrahlung)

If something carries the unit MeV, that means it has that amount of energy in it's constituent particles and is usually homogeneous in nature.

Photons produced from a machine such as an X-ray tube or a linear accelerator has an energy spectrum. For example, A 6 MV photon beam would constitute beams with energies ranging from 0 to 6 MeV. So, it has heterogeneous energy spectrum because of bremsstrahlung effects. Photons emitted from a radioactive decay would properly be considered MeV because they are mono-energetic.

The electrons emitted from a device is mono-energetic. All electrons have the same energy in MeV.