Is simple XOR with secret key using DHE secure? Client and server have given generator 3 and modulus which is 256 bit prime same all the time for all clients and server (simply hardcoded).

They both generate a private exponent and send result to each other and generate secure key. This all is done over insecure channel. That's all simple Diffie-Hellman exchange.

After this step every sent packet is encrypted over simple XOR block cipher by secure key's bytes, in this case 256bit long block.

Is this way of encryption any secure and if yes, how much, if no, what are ways to reverse it, as it is so simple and doesn't use much resources?
In addition to the problems that user595228 has mentioned, well, an attacker can easily solve a discrete log modulo a 256 bit prime; from that, he can recompute the shared secret, and that would give him the entire message.  To be secure, you really need at least a 1024 bit prime; with a 2048 bit prime being greatly preferred.