Although they're both part of the onion family and have similar leaf structures and growth habits, green onions and chives are entirely different species whose distinct characteristics lead them to fill very different roles in the home garden.
Green onion (Allium fistulosum) is a perennial that's hardy in U.S. Department of Agriculture hardiness zones 6 to 9, but it's often grown as an annual in colder zones. When left to grow as an perennial, it forms foot-wide clumps of upright leaves that are up to 2 feet tall, a habit that sometimes leads the plant to be called bunching onion. The plant is also commonly called scallion. Its leaves are tubular and hollow, and they remain green even through the winter.
Chives (Allium schoenoprasum) are perennials that are hardy in USDA zones 5 to 11, and they also can be grown as annuals outside their comfort zones. Like those of green onion, chive leaves are also tubular and hollow, but they're typically somewhat smaller both in diameter and height than green onion leaves. Chives grow in clumps that are about a foot wide, but although vigorous plants may reach 2 feet in height, they're more likely to stay closer to a foot high.
Green onions, like other varieties of onion, are grown for their edible bulbs. Green onion bulbs are smaller than those of bulbing varieties; green onion bulbs are elongated rather than round, and they are generally not much wider than the plant's stem. They also tend to have a milder flavor than the bulbs of other onion varieties.
The bulbs of chives are relatively insubstantial and don't have significant culinary value. Instead, chives are grown as a garden herb because their leaves have a mild onion flavor and work well as a seasoning in salads and other dishes. Leaves can be harvested at any time by cutting them off at the base of the plant.
The relatively fine texture and bright green color of chive leaves make them an attractive garden plant, and they have ornamental value beyond their value as a culinary herb. Plants also send up 18-inch-high flower stalks in the spring or summer that are crowned with attractive, globe-shaped, purple or white flower clusters.
Green onions are much less valuable as an ornamental. Their small, greenish flowers are typically less conspicuous and attractive than those of chives, and Allium fistulosum is rarely considered to be among the ornamental onion species.