According to Pokebeach, a distribution for Mew has been announced via the Japanese Pokemon website. Players who purchase the Virtual Console versions of Red, Green, Blue or Yellow between February 27th, 2016 and February 28th, 2017 will be able to download Mew into their X, Y, Omega Ruby or Alpha Sapphire games. Once you have downloaded Red, Green, Blue or Yellow, Mew can be downloaded into your Generation 6 games before March 31st, 2017.
In addition, four new 2DS bundles have been announced for release on the same day as the VC copies of the Gen 1 games, which match the color of the corresponding game. They will come with a pre-loaded copy of said color, (ie, the Green 2DS bundle gets Pokemon Green, etc.) and a serial number to download Mew into X, Y, Omega Ruby, or Alpha Sapphire, a map of the Kanto Region, a special Home screen design which features Ken Sugimori artwork, a set of stickers, and an “exchange ticket” which will allow you to get Mew in the old games.
Mew will come at Level 5 and only know Pound, and players will have to take their games to stores for the download, while the site says stores participating in the distribution will be announced at a later date.
Source: Pokebeach, The Pokemon Company