The words are not really comparable. They are rarely used as adjectives, actually. But when they are, their meaning can overlap or also not overlap.
"To boil" is primarily a verb. It denotes the process of cooking vegetables by immersing them in water at roiling boil.
"A stew" is primarily a noun. It denotes a kind of meal which has a liquid component, but not enough to count as a soup, has a mixture of many solid components in smallish pieces (you don't need a knife to eat it), and has been cooked for a long time, until everything is quite tender (especially if there is meat in it).
Preparing a stew could, in principle, involve boiling on stovetop. But tastewise, it turns out better if you use other techniques, such as simmering on stovetop or baking it in an oven.
"Stewed vegetables" is a construction which the language allows, but it's not really common. On a menu, you'll see "vegetable stew", not "stewed vegetables". If somebody would serve them nevertheless under this name, you'd really expect to get the whole stew (soft vegetables + liquid, as Kate Gregory said), and not only the vegetables alone.
"Boiled vegetables" is also rather unusual. It is the word a cook would use if a more specific one isn't applicable. For example, chefs rarely or never say "boiled vegetables" when using blanched vegetables, even though blanching is a special type of boiling. It is so unspectacular and connected to memories of bad cafeteria food that restaurants don't use it on a menu, frequently opting to leave the descriptor out altogether where it would have been applicable. You get "lamb ribs with peas and mint sauce", not "lamb ribs with boiled peas and mint sauce".
But because "boiled" is used when nothing more specific is available, if you happen to see it, you can really expect to get "naked" boiled vegetables, not stewed, creamed, or prepared by another method which involves boiling.