I agree with @darioo and most other answers. Always put readability (and maintainability) first. (A modern JIT compiler should have no troubles with simple cases like these.)
Here is however the bytecode corresponding to your program. (Note that the += approach results in a StringBuilder which is generally the preferred approach when constructing strings.)
// String hello = "hello";
0: ldc #2; //String hello
2: astore_1
// hello += "world";
3: new #3; //class java/lang/StringBuilder
6: dup
7: invokespecial #4; //Method StringBuilder.""
10: aload_1
11: invokevirtual #5; //Method StringBuilder.append
14: ldc #6; //String world
16: invokevirtual #5; //Method StringBuilder.append
19: invokevirtual #7; //Method StringBuilder.toString
// System.out.println(hello);
22: astore_1
23: getstatic #8; //Field System.out
26: aload_1
27: invokevirtual #9; //Method PrintStream.println
// String helloConcat = "hello ".concat("world");
30: ldc #2; //String hello
32: ldc #6; //String world
34: invokevirtual #10; //Method String.concat
37: astore_2
// System.out.println(helloConcat);
38: getstatic #8; //Field System.out
41: aload_2
42: invokevirtual #9; //Method PrintStream.println
45: return
