Make lemon curd: In a small saucepan over medium heat, whisk together 1⁄2 cup of the sugar, 1 egg, and the juice and zest of the lemon; cook, whisking constantly, until thickened, 8-10 minutes. Remove pan from heat and whisk in unsalted butter, letting each cube incorporate before adding the next. Strain curd through a fine sieve set over a small bowl; press plastic wrap against the surface of curd and refrigerate until well chilled.
Make meringue: Heat oven to 350°. In a small bowl, stir together cornstarch, vinegar, and vanilla extract; set aside. In the bowl of a stand mixer fitted with a whisk, beat remaining 2 1⁄2 cups of sugar and egg whites on low speed until combined. Increase speed to medium-high and beat until soft peaks form. Add cornstarch mixture to egg whites; continue beating until very stiff and glossy peaks form, about 5 more minutes.
Place a 9" round cake pan in the center of a 13" x 18" sheet of parchment paper and use a pencil to trace a circle around the outside of the pan. Flip the sheet of parchment paper and transfer it to a baking sheet so that the marked side is face down. Transfer meringue to the center of parchment paper.
Using a rubber spatula, shape it into a 9" disk by making the meringue conform to the circular outline; smooth top and sides with rubber spatula. Transfer meringue to the oven; reduce oven temperature to 215°; bake for 1 hour and 15 minutes. Turn off oven and let meringue sit until cooled, 3-4 hours. Gently peel parchment paper from the meringue and, using 2 metal spatulas, transfer meringue to a cake stand.
In the bowl of a stand mixer fitted with a whisk, beat heavy cream and yogurt on medium-high speed until stiff peaks form. Pour the whipped cream mixture onto the cooled meringue and spread evenly over meringue's top using a rubber spatula.
Decorate the top of the pavlova with strawberries and kiwifruit. Remove the reserved lemon curd from the refrigerator and stir vigorously; drizzle the curd over the pavlova, reserving a few tablespoons for individual servings. Cut the pavlova into slices and serve immediately with lemon curd.