Modern dirtbikes do not have sleeves. A sleeve is a round chunk of metal that sits in the cylinder that can be bored oversize when it wears. Modern dirtbikes have the aluminum cylinder walls plated with Nickel or Chrome or a combination thereof called NiChrom. This plating is much stronger and lasts for a very long time. As long as it looks ok when you replace the piston, you can keep using it.
certain years of kx's have their cylinders plated with something else through a process called electrofusion. It has a tendancy to flake away, so eventually needs to be replaced. If it is bad, you can either have a sleeve inserted into the cylinder, or have it plated with NiChrom. The sleeve option does not transfer heat as well, but allows you to overbore it when it wears (which is MUCH cheaper.) If you send it in to get plated, expect to pay upwards of $200. Plating transfers heat better than the sleeve, and lasts for a very long time, but cannot be overbored. Once it wears out, you have to send it back in to get replated again.
Some of this info may be slightly off, due to the fact that I have never owned a bike with a plated cylinder, and I learned all this while researching on a new bike I wanted to get, but most of it should be 100% correct...