As @Slovakov said, these kind of protections are never seen in XC, the main reason being that XC is a low risk activity. You usually go at slower paces and you never put yourself in situations where there is a likely chance you can loose control and have a high speed crash in a dangerous zone (as you do in downhill or freeride, for example).
Of course you can fall and hurt yourself, but you can fall and hurt yourself if you are trail running, you can twist an ankle if you are playing basketball, or hurt a tendon weightlifting. You have to understand physical activity isn't without a certain degree of risk.
The only reason why helmets are highly recommended is because, even in the unlikely event of having a crash, an impact in your skull can have huge consequences for yourself - much more than any other part of your body.
Also, if you are concerned with your safety, it is my belief training yourself and learning to ride a bike in a technical situation is a very valuable skill. Lower your saddle, move your ass right to the back, learn to control and feel your brakes, etc. Start with easy areas and progressively advance to places which are more rocky and have more inclination. Being comfortable around these situation plays a big part in your safety.
With that said, if you still want knee protection, the most lightweight I know of are POC VDP Air Knee and Elbow Pads, Dainese Trail Skins or Race Face Charge Leg Guards. Six Six One also makes pads with various densities. You will easily find all of these with a Google search.
Have fun and safe riding :)