Question: I just need a hello world demo to see how machine code actually works.

Though windows' **EXE** and linux' **ELF** is near machine code,but it's not **PURE**

How can I write/execute **PURE** machine code?

Answer: 
The OS is not running the instructions, the CPU does (except if we're talking about a virtual machine OS, which do exist, I'm thinking about Forth or such things). The OS however does require some metainformation to know, that a file does in fact contain executable code, and how it expects its environment to look like. ELF is not just *near* machine code. It **is** machine code, together with some information for the OS to know that it's supposed to put the CPU to actually execute that thing.

If you want something simpler than ELF but \*nix, have a look at the a.out format, which is much simpler. Traditionally \*nix C compilers do (still) write their executable to a file called a.out, if no output name is specified.
When targeting an embedded system you can make a binary image of the rom or ram that is strictly the instructions and associated data from the program. And often can write that binary into a flash/rom and run it.

Operating systems want to know more than that, and developers often want to leave more than that in their file so they can debug or do other things with it later (disassemble with some recognizable symbol names). Also, embedded or on an operating system you may need to separate .text from .data from .bss from .rodata, etc and file formats like .elf provide a mechanism for that, and the preferred use case is to load that elf with some sort of loader be it the operating system or something programming the rom and ram of a microcontroller. 

.exe has some header info as well. As mentioned .com didnt it loaded at address 0x100h and branched there.

to create a raw binary from an executable, with a gcc created elf file for example you can do something like 

objcopy file.elf -O binary file.bin

If the program is segmented (.text, .data, etc) and those segments are not back to back the binary can get quite large. Again using embedded as an example if the rom is at 0x00000000 and data or bss is at 0x20000000 even if your program only has 4 bytes of data objcopy will create a 0x20000004 byte file filling in the gap between .text and .data (as it should because that is what you asked it to do).

What is it you are trying to do? Reading a elf or intel hex or srec file are quite trivial and from that you can see all the bits and bytes of the binary. Or disassembling the elf or whatever will also show you that in a human readable form. (objdump -D file.elf > file.list)