I have Paragons elbow protectors, they have similar for the knees. These are one of the lightest Ive seen – but they will only offer enough protection for slow speed tumbles, but are pretty comfortable. I only ride mostly flat / not high speed forest trails – techy roots and rocks actual Nordic forest stuff, so speeds are never high and no long descents. Good for when you just topple over.
https://www.alpinestars.com/collections/cycling-protections/products/paragon-lite-knee-protector
But for knees Ive used an older version of Troy Lee (these new ones seem more flexible, and have that D3O impact hardening stuff, the old ones are from before these materials, Ive had them for years and have another unused pair waiting) –
Stage Knee Guard Solid Black
I have the mid level knee guards and light elbow ones on pretty much always when riding trails. Most of the damage is usually from just hitting the ground when falling from slow speeds, and they work well for that.
The Alpinestars Paragon ones are totally flexible, and they can be rolled up into small pouch that takes very little room. Troy Lee ones cannot. The Paragons only protect the pad area – the fabric around it is just thin flexible stuff.