There are three types of jeans, as in normal cotton jeans but for bikers: partially lined, fully lined, and single layer.
Partially lined — jeans with an inner aramid lining which protects the key areas, usually the bum, hips, and knees.
Fully lined — jeans with an inner aramid lining cover covers everywhere, though maybe stopping short of the ankles because that is where the podtction is provided by boots.
Single layer – jeans with aramid woven into them, so there is no inner lining.
Kevlar is just one brand name for aramid.
Fully lined jeans usually offer the most protection but are hotter and heavier, what with being two layers. Partially lined ones allow more comfort by sacrificing safety in the areas with the least risk and are cheaper. Single layer jeans are the coolest and the lightest, what with being just one layer. But compared to partially lines ones they offer more protection in some areas and less in others, as the extra layer of normal material also adds protection.
But overall safety cannot be judged by the construction, some partially lined jeans can be no safer than a regular thick pair you can get for £20. And a single layer pair of jeans can be as safe as a fully lined one if you are willing to pay.
To be assured of protection look for them being CE certified.
The relevant standard is prEN 17092-3 and items which pass testing are also given a rating from A to AAA. A bit like SHARP ratings for helmets, anything with an A rating is approved as meeting a minimum safety standard, but some items offer more protection than others. And some items may meet the older EN 13595 standard.
You can read details on the standards here:
Just do not confuse items which are only described as being CE approved as meeting those standard, as that may just refer to the included armour. If the jeans themselves are approved then the standard must be included on a label which features someone squatting between two tyres.