When trying to clean stone, I'd suggest starting with the mildest method and then progress until you see results. My recommendation would be hot water with some dishwashing detergent mixed into it. Apply it to the area and use a stiff bristle brush to scrub down the pavers. You could try 30 Seconds 2L Outdoor Cleaner With Rapid Hose End Technology if you see no effect. Ensure to flush it away with plenty of fresh water after the cleaning process to avoid damaging the garden plants.
Sometimes rust stains can penetrate quite deeply into stone surfaces. If these methods are not successful, you can move onto more aggressive treatments. Once the cleaning has been completed, you might like to consider sealing to help prevent staining in the future. - MitchellMc
I think your best bet is to go with a specialty paver cleaner (bleach diluted in water can work too depending on the type of paver) and a stiff-bristled brush, then rinse with water. I'd steer clear of using the Karcher to rinse it as you don't want chemicals being blasted everywhere. - Jackson
If the pavers butt closely together, a high pressure cleaner is ideal, but it does depend on what type of crud you have ingrained into them. If you have black mould, it can penetrate deeply and be hard to shift.
I'd recommend diluting some liquid pool acid, apply it sparingly to the stubborn areas, let it stand for a few minutes, then rinse it with a fine spray from your hose until you feel that's all flushed away. Going over it again with the pressure cleaner should have it looking a treat. - Andy_Mann
I've just recently tested my Ryobi pressure washer with pavement and decking attachment. I am very impressed but I can only vouch for using it on bricks. After it's clean and dry, seal the paver and prevent more contamination from coming in. If you don't have a pressure washer I suggest having a look at Ryobi 2000psi Pressure Washer with pavement and deck attachment. - redracer01