Parker added: "Anthony Joshua is in for a huge shock".
Joshua will look to beat Parker in style to set up another massive heavyweight clash with WBC champion Deontay Wilder, who faces Luis Ortiz a few weeks earlier in NY. My entire existence is now devoted to proving what the boxing world already knows'. My promoters are focused on what they have to do and I know what I have to do - train hard, be mentally strong, be in great shape and put in a great performance.
"These fights aren't easy, cause there's a lot on the line, so respect to Team Parker for taking the challenge".
Joshua has fought just once since beating the now retired Wladimir Klitschko, which came in a 10th-round stoppage of Frenchman Carlos Takam in October past year - also in Cardiff - having been originally slated to face Kubrat Pulev only for injury to prevent the Bulgarian from competing.
The American, whose three-year reign as champion is the longest in the division, wants a fight with Joshua, calling the British Olympic gold medal victor out after beating Bermane Stiverne in November.
'It is a unification heavyweight championship fight. "Being involved in a big fight is exciting - it's going to be a little part of history".
The last undisputed heavyweight champion of the world was Lennox Lewis in 1999.
Parker's promoter David Higgins predicted that his fighter will ruin the flawless record of Joshua.
Hearn said: "I'm delighted to get this fight made, it's been a long time coming".
"Joseph Parker is the absolute epitome of the Kiwi sporting battler".
Parker's last fight was in England when he defended his WBO title for the second time, beating Britain's Hughie Fury on points in Manchester in September.
Parker will fight Joshua at the Principality Stadium where he beat Carlos Takam in October 2017.
"He's never been stopped, never even been wobbled in the ring". Afterwards it was a case of "when" not "if" boxing would return and we are thrilled to be working with Matchroom once again to raise the bar for live boxing entertainment on March 31st.