Photo-via technology uses a  solder mask type dielectric to build up separate
conductor layers.  The manufacturing steps are as follows:
1. Start with a double side or simple multilayer board, with copper traces.
2. Coat photosensitive dielectric much the same as Photo Imageable Solder
Mask (PISM) by screen, curtain, dry film lamination. Dry if using a liquid.
3. Expose dielectric using via artwork.  
4. Develop out unexposed material under black areas of artwork.  This step
makes all the "blind" vias at once, in contrast to mechanical or laser
drilling.
5. Metallize the surface by additive techniques - electroless copper is
usually used to start a new conductive layer
6. Build up copper thickness to desired amount by plating
7. Apply temporary photoresist to define copper conductors in new conductor
plane, and to protect copper in photovias.
8. Etch
9. Strip temporary resist.
10. As needed by board complexity,  add more conductor layers by repeating
steps 2-9.

The technology is similar to semiconductor metallization using silicon
dioxide and aluminum. There are variations in the sequence of metallization
and plating, depending on company making the "build-up"  circuits.
 Advantages of technology are to make smaller vias than can be drilled and to
make thousands of vias at once. Need more detail - call me at 203-575-5740,
Denny Fritz, MacDermid.