What is the difference between the SHU, PC and a DO?

There is really no difference. They are all segregated custody from either an Administrative Order (AO) or a Disciplinary Order.(DO). The SHU is short for Special Housing Unit, Security Housing Unit or Segregated Housing Unit. and PC is "protective custody".

It sounds like something happened that caused the administration to declare him for PC on an DO.  We interpret this to mean he was threatened (or threatened someone else) and it is being determined is he is a safety issue in the general population. The only place an inmate can go is a cell where no one can get to him (or vice versa) is SHU. It is 23 hours a day lock-down, three showers a week and two days of one hour of fresh air in a concrete box with fencing over the top. SHU inmates have limited commissary, limited phone privileges and limited visitation. It is likely he will be transferred to a different facility where he can rejoin the general population.