In hyperventilation:
Hyperventilation washes out CO2 from body, leadng to alkalosis.So, Oxygen hemoglobin dissociation  curve will  shift  to  left  (high  affinity), decreasing P50 of hemoglobin.
Reference: Ganong’s Review of Medical Physiology 24th edition pg 611
P50 and Hb affinity for O2 increases
P50 and Hb affinity for O2 decreases
P50 increases and O2 affinity decreases
P50 decreases and O2 affinity increases