Why is ISS photo-voltaic current output shown as a negative value?
If this Reddit thread is to be believed, the currents are representing the current draw of the ISS batteries. Negative indicates that the solar panels are charging the batteries (driving current into the battery).



Looking at the Spartan Console handbook (the system from which you are looking at data on ISS Live), we can see that the data we are looking at is from each of the 8 battery controller channels:


  In order for the ISS to continually maintain electrical power, the SPARTAN flight controller oversees a set of batteries for each of the eight independent power “channels” (one for each solar array).


Current measurements taken for a battery will have one sign represent charging, and another sign representing discharging. As to which one is which, you would need more information.

Assuming the data for each is up to date, then at the moment of writing this answer:


ISSLive shows a negative current
ISSTracker shows that the ISS is currently on the night-side of the planet


As the ISS is on the night side of the planet, it's solar panels will be dark. That means that it must currently be running on battery power, i.e. the batteries must be discharging. 

Based on this a negative sign probably indicates discharging of the batteries.