I wouldn't say 'much' lighter. Unless you're looking in the 14'+ range, the scant savings in weight is greatly overshadowed by the inferior service life and 'repairability'. PVC doesn't have half the abrasion resistance, is very susceptible to UV and petroleum damage (which is irreversible), and doesn't glue well at all. A properly glued Hypalon seam will be stronger (in terms of shear strength) than the surrounding fabric. PVC not so. With even casual care, Hypalon will outlive well cared for PVC many times over.
For an example of the weight difference, look at Mercury's 430HD: The PVC version is 254lbs, the Hypalon is 278lbs. So, on a 14'01" inflatable, you're only looking at a 24lb increase in Hypalon over PVC, or less than 10%. Throughout most brand's model lines that offer PVC or Hypalon in the same design and construction of the same model, you'll find that the difference in weight is right around 8-10%.
Like anything else, though, it's all a matter of intended use and care. If you're looking for an inflatable at a low price point that will only see occasional light service use, will be stored out of the sun when not in use, and service life and resale aren't an important consideration, PVC may be worth looking into. I'd advise great caution if a gasoline outboard is in the picture, though, as PVC is highly susceptible to damage from gasoline and petroleum products. It develops a 'set' to the fabric rather easily, as well, such as when it's left lying afloat with an outboard attached. (Hypalon will eventually do this as well, but it's not permanent like with PVC. My Avon had a bit of sag to it last year that was easily corrected by leaving it fully inflated upside down for a bit over the winter.)