In the finite volume method the velocity components are calculated on the cell faces. but we remark that:

1- The speed vectors are ploted in grid centers (in photon)!

2- In the PHI (or Result) file we have the values of velocity components in the cell center!

is it an interpolation ? what kind ?

In PHI or RESULT the values of U1, V1, W1 are staggered and pertain at centres of cell faces.

In PHOTON they are averaged to the cell centre by a straightforward arithmetic mean. This averaging in PHOTON can be switched off by SET VEC AV OFF. Strictly speaking this is a preferable option when you are using BFCs. When a staggered velocity resolute is not available (e.g. on a boundary), PHOTON makes an intelligent assumption about it. (NB The VR Viewer assumes zero on a boundary, although I gather this will improve in 3.4).