Here is what they mean:
P0121 Check Engine Light requested by the throttle unit
P1172 Long-term fuel trim (fuel mixture out of range)
P1238 Turbocharger (TC) control system flow Fault
It is hard to say what is causing these 3 codes to set. When a code is stored on a Volvo, and many other vehicles, it stores information such as how long ago it was set, how many times the fault has occurred etc. so it's hard to say without seeing this info which code is for real and which may have been in the system for a long time.
An experienced Volvo tech at a dealer may solve this issue in half the time as an inexperienced tech at another shop, but that's not always the case.
Be advised that there is an extended warranty on the throttle modules from Volvo, and they had a lot of problems. Only a dealer can perform recall work. See here for more info http://repairpal.com/volvo-s80-1999/common_problems
You can start by performing a visual check of the intake pipes and hoses and see if any are split or loose, this may be an intake leak.