Instead of getting into the technical aspects of the difference between the generations of night vision, I will mention what they mean to the consumer.
This is the oldest and, by far, the most affordable. Depending on the night, they have a range of 75 yards. The resolution is poorer than and not as bright as newer generations. They only operate in active mode and, therefore, make you easy to see by others with night vision. Bright light can cause distortion called “blooming” easier than newer generations. Shorter life expectancy, only roughly 1500 hours.
Gen 1 has been around since the 1960’s, so generation 2 is a big step up. Viewable distance is out to roughly 200 yards with higher resolution. Passive mode can be used, and gen 2 is less susceptible to blooming. Life expectancy is roughly three times as long as gen 1. There are several types of Gen 2, some much better than others.
Gen 3 is considered the gold standard of night vision and is what the military is currently using. Gen 3 offers the best resolution and is usable out to 300 yards, depending on the model and night conditions. Gen three also operates in passive mode. Life expectancy is 10,000+ hours.
Gen 4 is equivalent or better than most gen 3. However, I have read that gen 4 is more prone to failures due to the technology it uses.
For those of you who want the more technical terms, you can find that information here. There are also visual examples of the differences between all four generations.
One final note; the owner of Ready Made Resources let me know they are having a sale through July on the PVS-14 3RD GEN with a free weapon mount and shutter eye relief.
They are gen 3, so they are expensive, but I did a bit of searching and this price is hundreds cheaper than other stores. If you can afford it and are in the market, this is a good price!