From the manual:
Calculating Characteristic Impedance
• Each port in a structure being analyzed can be viewed as a cross-section of a transmission line.
• HFSS computes the characteristic impedance of each port in three ways — as Zpi, Zpv, and Zvi impedances. You have the option of specifying which impedance will be used in the renormalization calculations.
• For TEM waves, the Zvi impedance converges on the port's actual impedance and should be used.
• When modelling microstrips, it is sometimes more appropriate to use the Zpi impedance.
• For slot-type structures (such as inline or coplanar waveguides), Zpv impedance is the most appropriate.
• HFSS will always calculate Zpi impedance