Program data vector (PDV)is a logical area in memory where 
SAS builds a data set, one observation at a time. When a 
program executes, SAS reads data values from the input 
buffer (a logical area in memory into which SAS reads each 
record of raw data when SAS executes an INPUT statement) or 
creates them by executing SAS language statements. The data 
values are assigned to the appropriate variables in the 
program data vector. From here, SAS writes the values to a 
SAS data set as a single observation. When the DATA step 
reads a SAS data set, SAS reads the data directly into the 
program data vector.

To add to it, Input buffer is the staging area for non sas
input.

input buffer contains one record of Raw data.
PDV contains one observation of dataset.

input buffer is created at the time of reading raw data to 
store raw data and send to PDV.

pdv is though a logical concept only but helps u to
understand the most important i.e, THE DATA STEP.
Two things u need to understand the compilation Phase and
the Execution Phase.
While reading a Raw data file:-

1) Compilation Phase - it checks for syntax errors and
conversion of ur data step into the machine code along with
creation of two things viz. INPUT BUFFER and PDV.
At the end of compilation phase u will have these two things
with u (Input Buffer and PDV).
In PDV all the variables present in the INPUT statement in
the data step will be give place in ur PDV as per the length
mentioned by u.

2) Execution Phase - Before beginning of the this phase all
the variables will be initialized to missing(if character)
and periods(if numeric)[use "put _all_" statement to check
ur PDV status  in the log].
Then when the input statement is encountered for the first
time The first record from raw data file is moved from it to
ur INPUT BUFFER. Then one to one mapping is done and data is
then again moved to PDV(So u can can take input buffer as a
temporary storage area for ur data, one at a time). Then
when run statement is encountered, then the implicit OUTPUT
statement forces the read data row to the output data set.