Stacks Blockchain Networks

Krypton is a Stacks 2 testnet with a fixed, two-minute block time, called regtest. Regtest is generally unstable for regular use, and is reset often. See the regtest documentation for more information on using regtest.
Testnet (old Xenon) is the Stacks 2 public testnet, which runs PoX against the Bitcoin testnet. It is the full implementation of the Stacks 2 blockchain, and should be considered a stable testnet for developing Clarity smart contracts. See the testnet documentation for more information on the public testnet.
Mainnet is the fully functional Stacks 2 blockchain, see the Stacks overview for information on running a Stacks node, mining, stacking, and writing Clarity smart contracts.