What is the best cardio exercise for a person with knee injuries?

Avoid pounding. Its probably best to perform cardio exercises which will allow pounding or knee impact the best exercises would probably be cycling, elliptical and swimming.
Cardio with knee inj. Low impact, like exercising in a pool, using stationary cycle or rowing machine, some stair masters, . All of these can help train your heart without over stressing your knees.
Low impact activitie. Low impact activities such as swimming, elliptical, treadmill, walking in general.